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PubMed1988

Nucleoside transport in rat cerebral-cortical synaptosomes. Evidence for two types of nucleoside transporters.

The transport of [U-14C]uridine was investigated in rat cerebral-cortical synaptosomes using an inhibitor-stop filtration method. Under these conditions the rapid efflux of uridine from the synaptosomes is prevented and uridine is not significantly metabolized in the synaptosome during the first 1 min of uptake. The dose-response curve for the inhibition of uridine transport by nitrobenzylthioinosine (NBMPR) was biphasic: approx. 40% of the transport activity was inhibited with an IC50 (concentration causing half-maximal inhibition) value of 0.5 nM, but the remaining activity was insensitive to concentrations as high as 1 microM. Similar biphasic dose-response curves were observed for dilazep inhibition, but both transport components were equally sensitive to dipyridamole inhibition. Uridine influx by both components was saturable (Km 300 +/- 51 and 214 +/- 23 microM, and Vmax. 12 +/- 3 and 16 +/- 3 pmol/s per mg of protein, for NBMPR-sensitive and NBMPR-insensitive components respectively), and inhibited by other nucleosides such as 2-chloroadenosine, adenosine, inosine, thymidine and guanosine with similar IC50 values for the two components. Inhibition of uridine transport by NBMPR was associated with high-affinity binding of NBMPR to the synaptosome membrane (Kd 58 +/- 15 pM). Binding of NBMPR to these sites was competitively blocked by uridine and adenosine and inhibited by dilazep and dipyridamole, with Ki values similar to those measured for inhibiting NBMPR-sensitive uridine influx. These results demonstrate that there are two components of nucleoside transport in our rat synaptosomal preparation that differ in their sensitivity to inhibition by NBMPR. Thus conclusions regarding nucleoside transport in rat brain based only on NBMPR-binding activity must be viewed with caution.

Animals↗
PubMed2001

Functional characterization of the alpha-glucoside transporter Sut1p from Schizosaccharomyces pombe, the first fungal homologue of plant sucrose transporters.

Disaccharide transporters have not previously been identified in Schizosaccharomyces pombe. This is in contrast to Saccharomyces cerevisiae in which several maltose permeases belonging to the sugar porter (SP) family have been characterized. Here we report that a novel S. pombe gene, sut1+, encodes a proton-coupled disaccharide uptake transporter in the glycoside-pentoside-hexuronide (GPH):cation symporter family. Previously, members of the GPH family were restricted to bacteria and plants. The closest homologues of sut1+ are the sucrose uptake transporters (SUTs) from higher plants that transport sucrose with a higher affinity than maltose. The transport function of Sut1p was analysed by expression in S. cerevisiae. Sut1p was found to transport maltose with a Km of 6.5 +/- 0.4 mM and sucrose with a Km of 36.3 +/- 9.7 mM. Therefore, the substrate specificity of Sut1p from S. pombe is different from that of its plant homologues. Glucose repression of sut1+ at the transcriptional level is also consistent with a physiological function for Sut1p in maltose uptake. These results indicate that, unlike S. cerevisiae, S. pombe utilizes maltose transporters derived from a common ancestor with the plant SUTs.

Amino Acid Sequence↗
PubMed1998

[Pulse oximetry and capnography in intensive care transportation: combined use reduces transportation risks].

OBJECTIVE: Due to the growing number of diagnostic and therapeutical procedures intensive-care patients must be transported intra- and interhospitally more often. These transports are among the most critical events during intensive-care therapy, with a high incidence of potentially life-threatening mishaps [23]. The aim of this study was to evaluate the possible benefit of the combined application of pulse oximetry and capnometry for patient safety during transport. METHODS: In a prospective clinical study 48 mechanically ventilated patients were allocated at random in 2 main study groups, 24 patients were investigated during interhospital transportation with an ambulance car, the other 24 patients during intrahospital transports. They were classified according to APACHE II and TISS. Blood pressure, heart rate and arterial blood gases were measured at eleven selected times. Twelve randomly chosen patients out of each main study group were monitored additionally with pulse oximetry and capnometry. The results were compared using the Mann-Whitney-U test. P < or = 0.05 was considered significant. RESULTS: Thirty-four patients had a TISS more than 40. The mean APACHE II-Score was 14 +/- 5. The overall incidence of potentially life-threatening mishaps was 9. Six out of these 9 occurred in the 24 patients with additional monitoring and were immediately detected by pulse oximetry or capnometry. CONCLUSIONS: The combination of pulse oximetry and capnometry offers the possibility to detect potentially life-threatening problems in ventilated patients during transport. This allows for early therapeutical consequences and may help to reduce the risk of transports.

APACHE↗
PubMedDate not supplied

[Measuring vibrations of transport stress in premature and newborn infants during incubator transport].

BACKGROUND: Despite increasing numbers of centers for perinatology, transportation of newborns and premature infants can not totally be avoided for several obvious reasons. The following investigations were carried out as part of our quality control measures of the established neonatal transportation system, and were aimed on the optimization of a new neonatal transportation equipment resulting in reduction of transportation stress caused by acceleration forces. METHOD: The new system investigated consisted of a Volkswagen type T4 equipped with a Dräger incubator type 5400, which was mounted on a pneumatic patient lift. We measured acceleration forces in three axes (expressed as K-Wert) as well as over a spectrum of frequencies (1-80 Hz). Measurements were taken at different points of the transportation unit during simulated transports driving a predetermined route. After obtaining informed consent of the parents, one actual transport of a newborn was used for an additional point measurement at the newborn's head. RESULTS: The mean K-Wert was decreased by about 50% in the vertical axis between the chassis of the car and the incubator by activating the pneumatic patient lift. Without activating the lift the K-Wert increased by about 20% between the car's chassis and the incubator. The frequency analysis showed resonance effects between the different components of the system. However, by activating the patient lift, effective accelerations in the incubator were decreased to less than 0.1 m/s2 across the whole frequency spectrum evaluated. The single measurement at a newborn's head revealed similar acceleration forces at the head of the baby and under its head. CONCLUSIONS: Utilization of a pneumatic patient lift can reduce acceleration forces. However, our results show that each system (car, incubator, and its base) has to be investigated and optimized for this purpose as a unit. Optimization of the complete system is necessary not only before its primary use but also in regular intervals over the years. Sometimes, further improvements can be reached with minor modifications such as the exchange of worn out rubber buffers.

Acceleration↗
PubMed2000

Microbial iron transport via a siderophore shuttle: a membrane ion transport paradigm.

A mechanism of ion transport across membranes is reported. Microbial transport of Fe(3+) generally delivers iron, a growth-limiting nutrient, to cells via highly specific siderophore-mediated transport systems. In contrast, iron transport in the fresh water bacterium Aeromonas hydrophila is found to occur by means of an indiscriminant siderophore transport system composed of a single multifunctional receptor. It is shown that (i) the siderophore and Fe(3+) enter the bacterium together, (ii) a ligand exchange step occurs in the course of the transport, and (iii) a redox process is not involved in iron exchange. To the best of our knowledge, there have been no other reports of a ligand exchange mechanism in bacterial iron transport. The ligand exchange step occurs at the cell surface and involves the exchange of iron from a ferric siderophore to an iron-free siderophore already bound to the receptor. This ligand exchange mechanism is also found in Escherichia coli and seems likely to be widely distributed among microorganisms.

Aeromonas hydrophila↗
PubMed1998

Human monocarboxylate transporter 2 (MCT2) is a high affinity pyruvate transporter.

The transport of pyruvate and lactate across cellular membranes is an essential process in mammalian cells and is mediated by the H+/monocarboxylate transporters (MCTs). We have molecularly cloned and characterized a novel human monocarboxylate transporter, MCT2. The cDNA is 1,907 base pairs long and encodes a polypeptide of 478 amino acids with 12 predicted transmembrane domains. Human MCT2 is the product of a single gene that mapped to chromosome 12q13 by fluorescence in situ hybridization. The kinetic properties of human MCT2 fulfill the criteria to establish it as a H+/monocarboxylate transporter; however, the unique biochemical feature of human MCT2 is its high affinity for the transport of pyruvate (apparent Km of 25 microM), implying that it is a primary pyruvate transporter in man. Comparison of human MCT1 and MCT2 with regard to tissue distribution and RNA transcript variants disclosed substantial differences. Human MCT2 mRNA expression was restricted in normal human tissues but widely expressed in cancer cell lines, suggesting that MCT2 may be pre-translationally regulated in neoplasia. We found co-expression of human MCT1 and MCT2 at the mRNA level in human cancer cell lines, including the hematopoietic lineages HL60, K562, MOLT-4, and Burkitt's lymphoma Raji, and solid tumor cells such as SW480, A549, and G361. These findings suggest that the two monocarboxylate transporters, MCT1 and MCT2, have distinct biological roles.

Amino Acid Sequence↗
PubMed1983

Mechanism of uptake and retrograde axonal transport of noradrenaline in sympathetic neurons in culture: reserpine-resistant large dense-core vesicles as transport vehicles.

The uptake and retrograde transport of noradrenaline (NA) within the axons of sympathetic neurons was investigated in an in vitro system. Dissociated neurons from the sympathetic ganglia of newborn rats were cultured for 3-6 wk in the absence of non-neuronal cells in a culture dish divided into three chambers. These allowed separate access to the axonal networks and to their cell bodies of origin. [3H]NA (0.5 X 10(-6) M), added to the axon chambers, was taken up by the desmethylimipramine- and cocaine-sensitive neuronal amine uptake mechanisms, and a substantial part was rapidly transported retrogradely along the axons to the nerve cell bodies. This transport was blocked by vinblastine or colchicine. In contrast with the storage of [3H]NA in the axonal varicosities, which was totally prevented by reserpine (a drug that selectively inactivates the uptake of NA into adrenergic storage vesicles), the retrograde transport of [3H]NA was only slightly diminished by reserpine pretreatment. Electron microscopic localization of the NA analogue 5-hydroxydopamine (5-OHDA) indicated that mainly large dense-core vesicles (700-1,200-A diam) are the transport compartment involved. Whereas the majority of small and large vesicles lost their amine dense-core and were resistant to this drug. It, therefore, seems that these vesicles maintained the amine uptake and storage mechanisms characteristic for adrenergic vesicles, but have lost the sensitivity of their amine carrier for reserpine. The retrograde transport of NA and 5-OHDA probably reflects the return of used synaptic vesicle membrane to the cell body in a form that is distinct from the membranous cisternae and prelysosomal structures involved in the retrograde axonal transport of extracellular tracers.

Animals↗
PubMed1982

Titration of transport and modifier sites in the red cell anion transport system.

This work demonstrates the existence of titratable transport and modifier sites in the anion transport system of human red cells. Effects of alkaline extracellular pH on chloride exchange were studied up to pH 13 at 0 degrees C. The studies revealed two sets of reversible titratable groups. One set, having a pK of or approximately 11, appeared to be identical with the inhibitory halide-binding modifier site. Deprotonation of this site stimulated anion transport. The apparent dissociation constants of chloride and iodide at this modifier site were 0.3 and 0.06 M, respectively, and it was confirmed that the organic sulfonate NAP-taurine inhibits anion transport reversibly by a high-affinity interaction with halide-binding modifier sites at the extracellular side of the membrane. Other groups, with apparent pK of or approximately 12 at chloride concentrations above 0.1 M, were named as "transport sites" because transport function depended totally on their protonation. The apparent pK decreased when extracellular halide concentrations was lowered below 0.1 M. It was dependent of the intracellular chloride concentration, and was equally sensitive to extracellular pH of 13, was fully reversible. Hydroxyl ions were not transported to an appreciable extent by the anion exchange system. The pK values of both sets of groups make it likely that they are both arginyl residues, functioning as anion recognition sites similar to the role of functionally essential arginyl residues observed with numerous enzymes.

Biological Transport↗
PubMed1999

Increased transport of pteridines compensates for mutations in the high affinity folate transporter and contributes to methotrexate resistance in the protozoan parasite Leishmania tarentolae.

Functional cloning led to the isolation of a novel methotrexate (MTX) resistance gene in the protozoan parasite Leishmania. The gene corresponds to orfG, an open reading frame (ORF) of the LD1/CD1 genomic locus that is frequently amplified in several Leishmania stocks. A functional ORF G-green fluorescence protein fusion was localized to the plasma membrane. Transport studies indicated that ORF G is a high affinity biopterin transporter. ORF G also transports folic acid, with a lower affinity, but does not transport the drug analog MTX. Disruption of both alleles of orfG led to a mutant strain that became hypersensitive to MTX and had no measurable biopterin transport. Leishmania tarentolae MTX-resistant cells without their high affinity folate transporters have a rearranged orfG gene and increased orfG RNA levels. Overexpression of orfG leads to increased biopterin uptake and, in folate-rich medium, to increased folate uptake. MTX-resistant cells compensate for mutations in their high affinity folate/MTX transporter by overexpressing ORF G, which increases the uptake of pterins and selectively increases the uptake of folic acid, but not MTX.

Animals↗
PubMed1983

Neonatal transport in Georgia: implications for maternal transport in high-risk pregnancies.

We analyzed population-based data from the Georgia Neonatal Surveillance Network from 1974 to 1978 to determine the effect of the hospital of delivery on the neonatal mortality rate (NMR) of infants later admitted to neonatal intensive case units (NICUs). The NMR of 3,524 infants transported from primary centers to NICUs was significantly higher [relative risk (RR) = 2.1; 95% CL = 1.9 to 2.3] than that of 10,764 infants born in tertiary centers and admitted directly to an NICU. This effect persisted even after adjustment for birth-weight differences (RR = 1.6; 95% CL = 1.5 to 1.8). The relative risk in favor of delivery in a tertiary center increased with increasing birth weight. However, the proportion of infants of less than 1,000 gm surviving was higher for transported infants and increased with distance transported. This finding suggests that, at these very low birth weights, the hardiest infants were selectively transported. A surprising finding was the very low proportion of low-birth-weight infants delivered in primary centers and transported to NICUs (eg, 32% of infants weighing between 1,001 and 1,200 gm). Our findings support previous reports that delivery in a hospital with an NICU is preferable to later neonatal transport and suggest that efforts to increase the rate of maternal transport in high-risk pregnancies can lead to a substantial reduction in infant deaths.

Adolescent↗
PubMed2004

Mercuric conjugates of cysteine are transported by the amino acid transporter system b(0,+): implications of molecular mimicry.

Humans and other mammals continue to be exposed to various forms of mercury in the environment. The kidneys, specifically the epithelial cells lining the proximal tubules, are the primary targets where mercuric ions accumulate and exert their toxic effects. Although the actual mechanisms involved in the transport of mercuric ions along the proximal tubule have not been defined, current evidence implicates mercuric conjugates of cysteine, primarily 2-amino-3-(2-amino-2-carboxyethylsulfanylmercuricsulfanyl)propionic acid (Cys-S-Hg-S-Cys), as the most likely transportable species of inorganic mercury (Hg(2+)). Because Cys-S-Hg-S-Cys and the amino acid cystine (Cys-S-S-Cys) are structurally similar, it was hypothesized that Cys-S-Hg-S-Cys might act as a molecular mimic of cystine at one or more of the amino acid transporters involved in the luminal absorption of this amino acid. One such candidate is the Na(+)-independent heterodimeric transporter system b(0,+). Therefore, the transport of Cys-S-Hg-S-Cys and cystine was studied in MDCK II cells that were or were not stably transfected with b(0,+)AT-rBAT. Transport of Cys-S-Hg-S-Cys and cystine across the luminal plasma membrane was similar in the transfected cells, indicating that Cys-S-Hg-S-Cys can behave as a molecular mimic of cystine at the site of system b(0,+). Moreover, only the b(0,+)AT-rBAT transfectants became selectively intoxicated during exposure to Cys-S-Hg-S-Cys. These findings indicate that system b(0,+) likely contributes to the nephropathy induced by Hg(2+) in vivo. These data represent the first direct molecular evidence for the participation of a specific transporter in the luminal uptake of a large divalent metal cation in proximal tubular cells.

Amino Acid Transport Systems↗
PubMed1979

Methionine transport in Salmonella typhimurium: evidence for at least one low-affinity transport system.

The systems which transport methionine in Salmonella typhimurium LT2 have been studied. Fourteen mutants, isolated by three different selection procedures, had similar growth characteristics and defects in the specific transport process showing a Km of 0.3 microM for L-methionine, and therefore lack the high-affinity, metP transport system. The sites of mutation in four of the mutants were shown by P1-mediated transduction to be linked (0.3 to 1.1%) with a proline marker located at unit 7 on the S. typhimurium chromosome. The high-affinity system was subject to both repression and transinhibition by methionine, and it may also be regulated by the metJ and metK genes. There appeared to be at least two additional transport systems with relatively low affinities for methionine in the metP763 mutant strain, with apparent Km values for methionine of 24 microM and approximately 1.8 mM. The latter system, with a very low affinity for methionine, was inhibited by leucine. In addition, methionine inhibited leucine transport, suggesting that one of the low-affinity methionine transport systems may actually be a leucine transport system.

Amino Acids↗
PubMed1978

Nucleoside transport in sheep erythrocytes: genetically controlled transport variation and its influence on erythrocyte ATP concentrations.

1. The permeability of sheep erythrocytes to purine and pyrimidine nucleosides was investigated. Erythrocytes from most sheep (nucleoside-impermeable) were almost completely impermeable to 5 mM inosine whereas cells from approximately 5% of the animals studied (nucleoside-permeable) showed a rapid inosine uptake. Cells from both types of animal were permeable to 5 mM adenosine, although transport was slower in nucleoside-impermeable erythrocytes. 2. Two distinct nucleoside transport routes were present in nucleoside-permeable erythrocytes; a high affinity (apparent Km congruent to 0.2 mM) facilitated diffusion system which transported both purine and pyrimidine nucleosides, and a non-saturable uptake route selective for adenosine. The high affinity system was the major route of adenosine transport at physiological concentrations. 3. Transport by the high affinity system was completely inhibited by micromolar concentrations of dipyridamole and nitrobenzylthioinosine. Dipyridamole had no effect on the non-saturable component of adenosine uptake. 4. The transport differences between nucleoside-permeable and impermeable erythrocytes were due to the absence of the high affinity system from nucleoside-impermeable cells. 5. Nucleoside-permeable cells had a higher intracellular ATP concentration than nucleoside-impermeable erythrocytes, suggesting that the high affinity transport system participates in the energy metabolism of the cell.

Adenosine Triphosphate↗
PubMed1977

Escherichia coli transport mutants lacking binding protein and other components of the branched-chain amino acid transport systems.

Further evidence on the role of binding proteins in branched-chain amino acid transport in Escherichia coli was obtained by selecting mutants with altered expression of the binding proteins. The mutator phage Mu was used to induce E. coli L-valine-resistant mutants defective in branched-chain amino acid transport. By making use of mild selective conditions and strain backgrounds with derepressed high-affinity, binding protein-mediated transport systems, we were able to isolate a new class of transport mutants defective in these systems. Mutant strains AE84084 (livK::Mucts) and AE840102 (livJ) were found to be defective in leucine-specific and LIV binding proteins, respectively, by transport assay, in vitro binding activity, and by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Mutant strain AE4107 (livH::Mu), although lacking high-affinity, branched-chain amino acid transport, retained functional binding proteins and therefore evidently codes for an additional component of high-affinity transport. The livH, livJ, and livK mutations were mapped by transduction and shown to be closely linked to each other in the malT region (min 74) of the E. coli genetic map.

Amino Acids↗
PubMed1989

Magnesium transport in Salmonella typhimurium: 28Mg2+ transport by the CorA, MgtA, and MgtB systems.

Three loci in Salmonella typhimurium (corA, mgtA, and mgtB) code for components of distinct Mg2+ transport systems (S. P. Hmiel, M. D. Snavely, J. B. Florer, M. E. Maguire, and C. G. Miller, J. Bacteriol. 171:4742-4751, 1989). Strains carrying one wild-type and two mutant alleles of the three loci were constructed to study the kinetics and specificity of ion transport of each system in isolation. The transport systems had different Km and Vmax values for Mg2+ uptake, and each was inhibited by other divalent cations in a distinct rank order of potency: for CorA, Mg2+ greater than Mn2+ greater than Co2+ greater than Ni2+ greater than Ca2+; for MgtA, Zn2+ greater than or equal to Mg2+ greater than Ni2+ approximately Co2+ greater than Ca2+; and for MgtB, Mg2+ approximately Ni2+ approximately Ni2+ greater than Mn2+ much greater than Ca2+. Other differences among the three systems were apparent. The CorA transport system functioned as a Mg2+-Mg2+ exchange system, mediating both efflux and influx of Mg2+. Neither the MgtA nor the MgtB system could mediate Mg2+ efflux. Transport via the MgtB system was very temperature sensitive; Mg2+ was transported at 37 degrees C but not at 20 degrees C. The MgtA and the MgtB transport systems were found to be regulated by the extracellular concentration of Mg2+.

Biological Transport↗
PubMed2001

In vivo synthesis of the periplasmic domain of TonB inhibits transport through the FecA and FhuA iron siderophore transporters of Escherichia coli.

The siderophore transport activities of the two outer membrane proteins FhuA and FecA of Escherichia coli require the proton motive force of the cytoplasmic membrane. The energy of the proton motive force is postulated to be transduced to the transport proteins by a protein complex that consists of the TonB, ExbB, and ExbD proteins. In the present study, TonB fragments lacking the cytoplasmic membrane anchor were exported to the periplasm by fusing them to the cleavable signal sequence of FecA. Overexpressed TonB(33-239), TonB(103-239), and TonB(122-239) fragments inhibited transport of ferrichrome by FhuA and of ferric citrate by FecA, transcriptional induction of the fecABCDE transport genes by FecA, infection by phage phi80, and killing of cells by colicin M via FhuA. Transport of ferrichrome by FhuADelta5-160 was also inhibited by TonB(33-239), although FhuADelta5-160 lacks the TonB box which is involved in TonB binding. The results show that TonB fragments as small as the last 118 amino acids of the protein interfere with the function of wild-type TonB, presumably by competing for binding sites at the transporters or by forming mixed dimers with TonB that are nonfunctional. In addition, the interactions that are inhibited by the TonB fragments must include more than the TonB box, since transport through corkless FhuA was also inhibited. Since the periplasmic TonB fragments cannot assume an energized conformation, these in vivo studies also agree with previous cross-linking and in vitro results, suggesting that neither recognition nor binding to loaded siderophore receptors is the energy-requiring step in the TonB-receptor interactions.

Bacterial Outer Membrane Proteins↗
PubMed1986

Incomplete nucleoside transport deficiency with increased hypoxanthine transport capability in mutant T-lymphoblastoid cells.

From a mutagenized population of wild-type mouse (S49) T-lymphoma cells, a clone, 80-5D2, was isolated in a single step by virtue of its ability to survive in 80 nM 5-fluorouridine. Unlike previously isolated nucleoside transport-deficient cell lines (A. Cohen, B. Ullman, and D. W. Martin, Jr., J. Biol. Chem. 254:112-116, 1979), 80-5D2 cells were only slightly less sensitive to growth inhibition by a variety of cytotoxic nucleosides and were capable of proliferating in hypoxanthine-amethopterin-thymidine-containing medium. The molecular basis for the phenotype of 80-5D2 cells was incomplete deficiency in the ability of the mutant cells to translocate nucleosides across the plasma membrane. Interestingly, mutant cells were more capable than wild-type cells of transporting the nucleobase hypoxanthine. Residual transport of adenosine into 80-5D2 cells was just as sensitive to inhibition by nucleosides and more sensitive to inhibition by hypoxanthine than that in wild-type cells, indicating that the phenomena of ligand binding and translocation can be uncoupled genetically. The 80-5D2 cells lacked cell surface binding sites for the potent inhibitor of nucleoside transport p-nitrobenzylthioinosine (NBMPR) and, consequently, were largely resistant to the physiological effects of NBMPR. However, the altered transporter retained its sensitivity to dipyridamole, another inhibitor of nucleoside transport. The biochemical phenotype of the 80-5D2 cell line supports the hypothesis that the determinants that comprise the nucleoside carrier site, the hypoxanthine carrier site, the NBMPR binding site, and the dipyridamole binding site of the nucleoside transport function of mouse S49 cells are genetically distinguishable.

Animals↗
PubMed1968

Renal tubular transport of proline, hydroxyproline, and glycine. 3. Genetic basis for more than one mode of transport in human kidney.

Impaired renal tubular transport of proline, hydroxyproline, and glycine was inherited as an autosomal recessive trait in two Ashkenazi-Jewish pedigrees and one French-Canadian family; the heterozygotes for the trait exhibited hyperglycinuria only. Intestinal transport of imino acids and glycine was not impaired in homozygotes. It is possible that more than one mutant allele may occur at a locus controlling tubular transport of the imino acids and glycine, since one subject with the imino-glycinuric phenotype had one parent who was not hyperglycinuric. More than 60% of the specific tubular transport function is still available in homozygotes for absorption of imino acids and glycine at endogenous substrate concentrations; however, this persistent transport is already saturated at these concentrations in contrast to the large capacity available in normal subjects. Furthermore, the glycine portion of this persistent transport is noninhibitable by imino acids in contrast to the normal situation. The imino acids can inhibit each other's uptake in mutant and normal phenotypes. Two modes of transport for the imino acids and glycine are proposed to explain these observations: (1) a common system with high capacity, and (2) two additional systems, each with low capacity (one-tenth or less of the common system). One of these systems is apparently shared by proline and hydroxyproline. The mutant allele(s) observed in this investigation occur at the locus for the common system.

Adolescent↗