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Biomedical subjects

C Tiruppathi

Publications and source records attributed to C Tiruppathi.

At least 55 records · Page 3Linked to original sources
PubMed1989

Cobalamin release from intrinsic factor and transfer to transcobalamin II within the rat enterocyte.

To ascertain the mechanism of release of cobalamin (Cbl) from intrinsic factor (IF) and subsequent formation of transcobalamin II (TC-II)-Cbl complex, we studied the intracellular distribution of 57Co-labeled Cbl after its uptake in suckling and adult rats. The amount of Cbl bound to IF, to the IF-Cbl receptor via IF, and to TC-II was determined by immunoprecipitation with monospecific antisera raised to these proteins. IF-Cbl receptor activity was found to be very low in suckling rats up to 12 days after birth. Oral administration of leupeptin in amounts known to alter protein turnover had no effect on the release of Cbl from IF nor did it inhibit the formation of the TC-II-Cbl complex in either adult or suckling animals. However, oral administration of chloroquine resulted in a transient increase in the intestinal concentration of Cbl in both adult and suckling rats and in total inhibition of Cbl released from IF in adults rats. Chloroquine prevented completely the transfer of Cbl to TC-II in adult rats and inhibited the transfer by 50% in suckling rats. These data demonstrate that in adult mucosa utilizing receptor-mediated endocytosis, Cbl is transferred from IF to TC-II. This transfer does not require the IF-Cbl receptor, as it occurs in suckling rats. Finally, transfer of Cbl to TC-II is decreased by a drug that alters vesicular pH. Because Cbl can be released at acid pH from IF, it is proposed that release of Cbl from IF and its transfer to TC-II occurs in an acidic vesicle.

Administration, Oral↗
PubMed1989

Multiple transport systems for organic cations in renal brush-border membrane vesicles.

The characteristics of guanidine uptake in brush-border membrane vesicles isolated from rabbit renal cortex were investigated. Guanidine uptake was markedly stimulated by an outwardly directed H+ gradient, resulting in a transient uphill transport. This stimulation was not due to an inside-negative, H+-diffusion potential because an ionophore-induced H+-diffusion potential and a K+-diffusion potential (both inside-negative) failed to enhance guanidine uptake. The H+ gradient itself appeared to be the driving force for the uptake. These data suggest that guanidine-H+ antiport (or guanidine-OH- symport) is the mechanism of guanidine uptake in these membrane vesicles. Guanidine uptake was only minimally inhibited by organic cations such as tetraethylammonium, N1-methylnicotinamide, and choline, but many other organic cations such as amiloride, clonidine, imipramine, and harmaline caused considerable inhibition. Uptake of radiolabeled guanidine was inhibited more effectively by guanidine than by tetraethylammonium, whereas uptake of radiolabeled tetraethylammonium was inhibited more effectively by tetraethylammonium than by guanidine. beta-Lactam antibiotics did not inhibit guanidine uptake but did inhibit tetraethylammonium uptake. Kinetic analysis showed that there were at least two kinetically distinct carrier systems for guanidine uptake, whereas tetraethylammonium uptake occurred via a single carrier system. These data provide evidence that renal brush-border membranes possess multiple carrier systems for organic cations.

Animals↗
PubMed1989

Inactivation of the human placental serotonin transporter by tyrosyl group-specific reagents.

Treatment of human placental brush-border membrane vesicles with tyrosyl group-specific reagents, N-acetylimidazole, 7-chloro-4-nitrobenzo-2-oxa-1,3,-diazole and tetranitromethane, inhibited NaCl gradient-driven serotonin uptake in these vesicles without affecting vesicle integrity. The concentrations of these reagents causing 50% inhibition of serotonin uptake were 3.75 mM, 10 microM and 5 microM, respectively. The inhibition of N-acetylimidazole was reversible with hydroxylamine and the inhibition by 7-chloro-4-nitrobenzo-2-oxa-1,3-diazole was reversible with 2-mercaptoethanol. Kinetic analysis of serotonin uptake in control and in N-acetylimidazole-treated membrane vesicles revealed that the treatment decreased the maximal velocity of the transport system with virtually no effect on the affinity of the transporter for serotonin. Similarly, the treatment did not change the affinity of the transporter for Na+. Even though serotonin uptake was reduced in treated vesicles compared with control vesicles at all concentrations of Na+, in both cases the dependence of the uptake rate on Na+ concentration was hyperbolic, indicating the involvement of one Na+ per transport of one serotonin molecule. The serotonin transporter could be protected from the N-acetylimidazole-induced inhibition by Na+. It is concluded that tyrosyl residues are essential for optimal transport function of the human placental serotonin transporter and that these critical tyrosyl residues are located at or near the Na+-binding site.

4-Chloro-7-nitrobenzofurazan↗
PubMed1988

A proton gradient, not a sodium gradient, is the driving force for active transport of lactate in rabbit intestinal brush-border membrane vesicles.

An inward-directed H+ gradient markedly stimulated lactate uptake in rabbit intestinal brush-border membrane vesicles, and uphill transport against a concentration gradient could be demonstrated under these conditions. Uptake of lactate was many-fold greater in the presence of a H+ gradient than in the presence of a Na+ gradient. Moreover, there was no evidence for uphill transport of lactate in the presence of a Na+ gradient. The H+-gradient-dependent stimulation of lactate uptake was not due to the effect of a H+-diffusion potential. The uptake process in the presence of a H+ gradient was saturable [Kt (concn. giving half-maximal transport) for lactate 12.7 +/- 4.5 mM] and was inhibited by many monocarboxylates. It is concluded that a H+ gradient, not a Na+ gradient, is the driving force for active transport of lactate in rabbit intestinal brush-border membrane vesicles.

4,4'-Diisothiocyanostilbene-2,2'-Disulfonic Acid↗
PubMed1988

Fatty acid-induced alterations in transport systems of the small intestinal brush-border membrane.

We have investigated the effects of fatty acids on the Na+-H+ exchanger and other carrier-mediated transport systems in intestinal brush-border membrane vesicles. The Na+-H+ exchanger (i.e. H+ gradient-dependent, dimethylamiloride-sensitive Na+ uptake) was strongly inhibited by fatty acids and the inhibition was concentration dependent. Unsaturated fatty acids showed more inhibition than saturated fatty acids. Among unsaturated fatty acids, ricinoleic acid was found to be the most potent inhibitor. Inhibition of the Na+-H+ exchanger by oleic acid was partially reversible, and the nature of the inhibition was found to be non-competitive with respect to Na+. The dimethylamiloride-sensitive Na+ uptake measured in the absence of an H+ gradient was also inhibited by oleic acid, suggesting that the inhibition of the Na+-H+ exchanger by fatty acids was not due to the accelerated dissipation of the H+ gradient. Treatment of the membrane vesicles with oleic acid also inhibited other carrier-mediated transport systems as well, such as the H+ gradient-driven transport of glycylsarcosine and the Na+ gradient-driven transport of D-glucose and L-alanine, whereas it did not affect the permeability of L-glucose, a non-carrier-mediated process. However, the inhibitory effects of oleic acid on the transport of D-glucose and L-alanine appeared to be related to the enhanced collapse of the Na+ gradient rather than a direct effect on the carrier systems because transport of these solutes when measured in the absence of a Na+ gradient ([Na+]i = [Na+]o) was not affected by oleic acid. These data demonstrate that fatty acids bring about significant alterations in the activities of various transport systems of the small intestinal brush-border membrane, either by directly interacting with the transport protein or by abolishing the energy source that is necessary for the transport process.

Animals↗
PubMed1988

Sodium-gradient-driven, high-affinity, uphill transport of succinate in human placental brush-border membrane vesicles.

Brush-border membrane vesicles isolated from normal human term placentas were shown to accumulate succinate transiently against a concentration gradient, when an inward-directed Na+ gradient was imposed across the membrane. This uptake was almost totally due to transport into intravesicular space, non-specific binding to the membranes being negligible. The dependence of the initial uptake rate of succinate on Na+ concentration exhibited sigmoidal kinetics, indicating interaction of more than one Na+ ion with the carrier system. The Hill coefficient for this ion was calculated to be 2.7. The Na+-dependent uptake of succinate was electrogenic, resulting in the transfer of positive charge across the membrane. Kinetic analysis showed that succinate uptake in these vesicles occurred via a single transport system, with an apparent affinity constant of 4.8 +/- 0.2 microM and a maximal velocity of 274 +/- 4 pmol/20 s per mg of protein. Uptake of succinate was strongly inhibited by various C4 or C5 dicarboxylic acids, whereas monocarboxylic acids, amino acids and glucose showed little or no effect. Li+ and K+ could not substitute for Na+ in the uptake process. Instead, Li+ was found to have a significant inhibitory effect on the Na+-dependent uptake of succinate.

Biological Transport, Active↗
PubMed1987

The role of choline on the activity-temperature relationship of brush-border alkaline phosphatase.

We have studied the effect of choline on the activity and temperature dependency of the brush-border alkaline phosphatase isoenzymes from rat intestine (tissue-specific type), and from kidney and placenta (tissue-nonspecific type). The removal of choline with phospholipase D resulted in the loss of enzyme activity in all the membranes, whereas in situ loss in the discontinuity of Arrhenius plots occurred in the kidney and the placental membranes, but not in the intestinal membranes. The lost activity was restored either by addition of free choline or phosphatidylcholine or by the removal of the enzyme from the membrane surface. Intestinal enzyme was removed by papain, while the tissue-nonspecific enzyme was released by subtilisin and by phosphatidylinositol-specific phospholipase C. The enzyme from kidney and placental membranes aggregated (rho = 1.13) upon removal of choline, and addition of choline resulted in disaggregation (rho = 1.03). Conversion of discontinuous to continuous linear plots of alkaline phosphatase in the kidney and placental membranes paralleled the increase in membrane phosphatidic acid content, and the decrease in total phosphatidylcholines. The intestinal enzyme produced plots with break points at all phosphatidic acid/phosphatidylcholine ratios. The change brought about by treatment with phospholipidase D was not due to changes in the half-saturation kinetics (Km) for the substrate. Based on these studies we conclude that the active site of the tissue-nonspecific phosphatase is approximated to exterior membrane cholines, as in the case of the intestinal isoenzyme; that despite similar effects on the membrane content of phospholipids, phospholipase D treatment caused much greater effects on the tissue-nonspecific enzyme, as assessed by Arrhenius plots and density centrifugation; that these effects are due to different protein structures rather than to a lipid milieu unique to each brush-border membrane.

Alkaline Phosphatase↗
PubMed1987

Hydrophobic interactions of brush border alkaline phosphatases: the role of phosphatidyl inositol.

Tissue-specific (intestinal) and tissue-nonspecific (kidney) rat alkaline phosphatases are released from their respective brush border membranes by different enzymes. To elucidate the mechanism underlying their membrane attachment, we tested the ability of these enzymes to partition into lipid or aqueous phases both before and after treatment with phospholipases and proteases. Interaction with Triton X-114 micelles was eliminated or decreased by treatment of intestinal enzyme with phospholipase A2 or papain, while only phosphatidylinositol (PI)-specific phospholipase C (PIPLC) and subtilisin were effective with the kidney enzyme. Binding to octyl Sepharose for the intestinal enzyme was decreased by phospholipase A2 more than by PIPLC, whereas the reverse was true for the kidney enzyme. Treatment with phospholipases decreased the apparent mass of the phosphatases by 50-80 kDa, presumably due to loss of bound lipid and detergent. PIPLC treatment of the kidney, but not the intestinal enzyme, prevented binding of the phosphatase to phospholipid vesicles. These results show that both enzymes are bound to respective membranes by hydrophobic anchor peptides to which phospholipids are bound. However, their sensitivity to phospholipases is different. The data are consistent with the hypothesis that, in the kidney enzyme, the PI is bound covalently, while with the intestinal enzyme, binding of PI appears to be tight but not covalent.

Alkaline Phosphatase↗
PubMed1987

Development of dipeptide transport in rat renal brush border membranes: studies with glycylsarcosine.

Dipeptide-proton cotransport was studied in rat renal brush border membrane vesicles from animals of different ages, starting at 7 days after birth. In the presence of an inward-directed proton gradient, glycyl-sarcosine uptake exhibited the overshoot phenomenon in all age groups studied. The magnitude of the "overshoot" increased with age and maximum accumulation of glycyl-sarcosine was observed in 42-day-old rat renal brush border membrane vesicles. Kinetic studies with glycyl-sarcosine indicated changes only in maximal velocity without any significant change in the apparent affinity value during the postnatal development of the renal brush border membrane. Since the Na+-H+ exchanger located at the brush border membrane is primarily responsible for the generation of the proton gradient, the driving force for dipeptide transport, we also measured the Na+-H+ exchanger activity in rat renal brush border membrane vesicles from suckling and adult animals. The exchanger activity was significantly greater in the adult rats compared to the suckling rats. In addition, we have confirmed in the present study previous findings from other laboratories that the activities of the Na+ gradient-driven glucose and amino acid transport systems in renal brush border membrane vesicles were higher in the adult than in the suckling rats.

Age Factors↗
PubMed1987

Inhibition of brush-border membrane Na+-H+ exchanger by loperamide.

Loperamide, an opiate agonist, inhibited the Na+-H+ exchanger in brush-border membrane vesicles isolated from term human placenta, rabbit renal cortex and rabbit small intestine in a dose dependent manner. Because the placental Na+-H+ exchanger was the most sensitive to inhibition by loperamide (IC50 = 60 microM), characterization of the inhibition was done with the placental Na+-H+ exchanger. The inhibition of the placental Na+-H+ exchanger by loperamide was instantaneous and freely reversible. Kinetic analyses demonstrated that the inhibition was of a mixed type. Loperamide (70 microM) reduced the maximal velocity (Vmax) from 46.4 +/- 1.2 to 34.5 +/- 1.1 nmol/mg of protein/15 s and increased the apparent affinity constant (Kt) for Na+ from 12.3 +/- 1.0 to 16.5 +/- 1.5 mM. Loperamide interacted with the exchanger protein at more than one site. The effects of loperamide were not antagonized by naloxone, suggesting the noninvolvement of opiate receptors in this process. These results differentiate loperamide from other Na+-H+ exchanger inhibitors such as amiloride, cimetidine and clonidine, which interact with the exchanger at a single site in a strictly competitive manner.

Amiloride↗
PubMed1986

Phase separation of rat intestinal brush border membrane proteins using Triton X-114.

Rat intestinal microvillus membrane contains at least 24 polypeptides, of which 18 can be solubilized using Triton X-114 at 4 degrees C. Upon phase separation at 32 degrees C, 11 proteins separated nearly completely into the detergent-rich phase, while 9 proteins were found exclusively in the aqueous phase. Enzymes which were uniquely included in the detergent phase were alkaline phosphatase, leucine aminopeptidase, gamma-glutamyl transpeptidase, and Ca2+-Mg2+ ATPase. The proteins which were excluded from the detergent phase and found exclusively in the aqueous phase included the disaccharidases (glucoamylase, sucrase-isomaltase, trehalase, lactase) and the ileal receptor for the intrinsic factor-cobalamin complex. Integral membrane proteins can thus be separated during solubilization into two groups prior to further purification or characterization.

Animals↗
PubMed1985

Membrane interactions of rat intestinal alkaline phosphatase: role of polar head groups.

Lipid-protein interactions with purified membranous intestinal alkaline phosphatase have been studied by using rat intestine. The enzyme was incorporated equally well into neutral lecithin and anionic liposomes, including those made from phosphatidic acid alone. It could not be solubilized with chaotropic salts nor by phospholipases C and D from either native membranes or phospholipid vesicles. Detergents effected nearly complete release of enzyme from the vesicles. Phosphatase activity was lost upon treatment with phospholipase D alone. The activity was restored with free choline, or choline containing phospholipids, but not by the addition of other phospholipids or amines. The catalytic activity was also lower when the enzyme was bound to a phosphatidylcholine vesicle containing additional phosphatidic acid. Neither phosphatidylserine nor phosphatidylinositol addition altered enzyme activity. These results show that the enzyme binds to the membrane by a primary hydrophobic interaction with membrane phospholipids without requiring the polar head group and that the enzyme activity is affected via a secondary interaction with choline. We suggest that choline protects the active site of brush border alkaline phosphatase from inhibition by endogenous membrane phosphate groups.

Alkaline Phosphatase↗
PubMed1985

Interaction of intestinal disaccharidases with phospholipids: effect of cholesterol.

Although the rat intestinal brush border disaccharidases are the most easily solubilized protein components, the nature of the lipid-protein interactions in the membrane is incompletely understood. Phospholipid vesicles were prepared using the lecithin fraction from brush border membranes and synthetic lecithins. Addition of cholesterol to brush border lecithins enhanced the binding of disaccharidases, but not of alkaline phosphatase. The addition of cholesterol to synthetic lecithin vesicles enhanced the binding of disaccharidases only when added above the transition temperature of the lecithin used. The maximal effect occurred at an equimolar ratio of lecithin to cholesterol. Binding of disaccharidases to phospholipid vesicles was independent of charge or the nature of the polar head group, and the enzyme was inserted so that the catalytic domain was excluded from the lipid matrix. These results demonstrate that membrane attachment of disaccharidases is hydrophobic, involving primarily fatty acyl chains and an interaction with cholesterol. The membrane interaction does not seem to affect enzyme activity.

Animals↗
PubMed1985

Intestinal uptake and release of cobalamin complexed with rat intrinsic factor.

The mechanism of uptake of intrinsic factor (IF) and cobalamin (Cbl) by enterocytes and their subsequent fate have been uncertain. To examine this problem double-labeled IF X Cbl was added to small intestinal organ cultures. When 125I-IF X [57Co]Cbl was added to rabbit ileal explants, binding and internalization increased linearly for 24 h. After an 18-h chase with nonlabeled IF X Cbl, no 125I-IF returned to the cell surface. An amount of 35-45% of the internalized Cbl was found free, not bound to IF or any other protein. About 60% of both internalized ligands was bound to membranes but by a non-Ca2+-dependent bond, suggesting binding to a protein other than the brush-border receptor. Cobalamin was released from IF at pH 5.0 to the same degree (30%) as free Cbl was found inside the cell (35-45%). Neither pancreatic proteases nor ileal homogenates effected release of Cbl from IF. When cathepsins were added, the Cbl released was no greater than could be attributed to pH 5.0 alone. Chloroquine added to tissue explants did not alter the percentage of free intracellular Cbl. From these results we suggest that IF X Cbl is internalized and detached from the receptor within the enterocyte. The mechanism of release is not known but seems to require an acid pH (5.0). The Cbl is released in the mucosa, perhaps when the IF X Cbl complex enters a nonlysosomal cellular compartment with an acidic environment. There is no substantial recycling of IF to the brush-border membrane.

Animals↗
PubMed1983

Faecal free fatty acids in tropical sprue and their possible role in the production of diarrhoea by inhibition of ATPases.

Faecal excretion of fatty acids is increased in patients with tropical sprue because of unabsorbed dietary fatty acids. The excretion of fatty acids correlates well with faecal wet weight. In vitro unsaturated fatty acids inhibited Na K-ATPase and Mg-ATPase isolated from basolateral membranes of enterocytes and colonocytes. These findings are a possible explanation for the observed abnormalities in water and electrolyte absorption by the colon in patients with tropical sprue and steatorrhoea.

Adenosine Triphosphatases↗
PubMed1982

Purification and properties of an acid lipase from human gastric juice.

An acid lipase (EC 3.1.1.3) from human gastric juice was purified by using poly(ethylene glycol)-6000 precipitation, ethanol fractionation and Sephadex G-75 gel filtration. A molecular weight of 44000 was obtained by SDS-polyacrylamide gel electrophoresis. pH-dependent aggregation was observed and by using Sephadex G-200 gel filtration, a molecular weight of 90000 was obtained at pH 6.0 and 45000 at pH 3.0, for the purified enzyme. A pH optimum of 5.3 was obtained using triolein as substrate. The apparent Km for tributyrin and triolein was found to be 21 and 73 mumol, respectively. Diacylglycerol and free fatty acids were the major hydrolytic end products of this enzyme. Studies on the positional specificity of the enzyme showed that the preferred site of hydrolysis was sn-3 and sn-1, although a good percentage of the sn-2 position was also hydrolysed. Conjugated bile salts inhibited the enzyme when triolein was used as substrate, whereas they activated it when tributyrin was used. Some of the properties of the purified human gastric juice acid lipase resembles those of rat and human lingual lipase.

Bile Acids and Salts↗