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PubMed1986

r-Galactonolactone in experimental galactosemic animals.

An accumulation of galactose-1,4-lactone, an oxidation product of galactose, was observed in various tissues of galactosemic guinea pigs fed a 40% galactose diet for 6 weeks. In addition, an accumulation of the two galactose metabolites varied among organs. The highest content of the lactone was observed in the liver and the content of the lactone exceeded that of the reduced counterpart. The lens gave the highest galactitol content. In the serum the level of the lactone was very low. A trace amount of the lactone was detected in the kidney while it was mostly excreted into urine within 54 h upon withdrawal of the diet. On the other hand, in the animals kept on a high galactose diet for only 2 days, urinary lactone rapidly decreased. These observations indicated that a high galactose level in the circulation was associated with the production of the lactone in various tissues and that the accumulated lactone was released into the circulation very slowly and then excreted into the urine. Suppression of galactitol production by administration of an aldose reductase inhibitor resulted in the accumulation of the lactone in the lens, the testis, and the muscle, as well as in the circulation. The lactone thus produced was excreted exclusively into the urine. This observation indicates a close relationship between the oxidative and reductive metabolisms of galactose at a toxic level.

Aldehyde Reductase↗
PubMed1975

Zero-trans and infinite-cis uptake of galactose in human erythrocytes.

1. The zero-trans and infinite-cis uptake of galactose into human erythrocytes was measured as a function of galactose concentration at 20 degrees C. 2. A special procedure, the "cis-trans test" has been developed to determine the directionality of an asymmetric transport carrier. 3. Using the "cis-trans test" and results obtained by phloretin inhibition, could show the existance of two sites mediating galactose uptake. The kinetic parameters of the high affinity site are K1 equals 11 mM; V1 equals 16 mmol - cell unit-1-min-1 and of the low affinity site: K2 equals 286 mM; V2 equals 21 mmol-cell unit-2-min-1. 4. The infinite-cis Km, using an intergrated rate equation treatment was 21 mM and that found by a direct preloading procedure was 25 mM. The existence of a high affnity site at the inner side of the membrane was thus confirmed.

Biological Transport, Active↗
PubMed1975

Zero-trans and equilibrium-exchange efflux and infinite-trans uptake of galactose by human erythrocytes.

1. The zero-trans and equilibrium exchange efflux and the infinite-trans uptake of galactose in human erythrocytes were measured as a function of galactose concentration at 20 %. 2. The zero-trans procedure with cells loaded with 285 mM galactose revealed a low affinity site for galactose transport at the inner face of the membrane having a maximal velocity of 255 plus or minus 96 mmol/l isotonic cell water and Km equals 240 plus or minus 57, the V/K ratio being 1.01 plus or minus 0.04 min-1. 3. The equibirum-exchange procedure yielded a maximal velocity of 432 plus or minus 44 mmol/cell unit per min and K equals 138 plus or minus 57, the V/K ratio being3.19 plus or minus 0.52 min-1. 4. The infinite-trans uptake revealed a high affinity site at the outer face of the membrane having a maximal velocity of 239 plus or minus 11 mmol/cell unit per min, and K equals 21 plus or minus 2 mM. 5. These results combined with previous findings (Ginsburg, H. and Stein, W. D. (1975) Biochim. Biophys. Acta 000, 000-000) force us to reject the following models for sugar transport in human erythrocytes: a single asymmetric carrier; two symmetric carriers in parallel, the original form of the internal transfer model.

Biological Transport, Active↗
PubMed1978

Galactose transport in human erythrocytes. The transport mechanism is resolved into two simple asymmetric antiparallel carriers.

The kinetic properties of the mediated transport of galactose in human erythrocytes are investigated at 20 degrees C. Different methodological procedures are used to acquire a complete kinetic description of the system. Under zero-trans conditions the uptake of galactose is mediated by two distinctly different carriers (defined as alpha and beta) having significantly different Mic;aelis parameters: alpha K = 12.7 mM and beta K = 81.5 mM, but similar maximal velocities, approx. 40 nM.min-1. The zero-trans efflux procedure reveals apparently one single carrier with K = 74.4 mM and V = 241 mM.min-1. Under equilibrium-exchange conditions the galactose transport is mediated apparently by a single site with K = 146 mM and V = 521 mM.min-1. The data for the alpha-carrier are analyzed in terms of the simple carrier model as formulated by Lieb and Stein (Biochim. Biophys. Acta (1974) 373, 178). Application of several rejection criteria for the simple carrier failed to indicate lack of fitness of the alpha-carrier to a simple asymmetric carrier. From the analysis of the kinetic data it is inferred that the transport of galactose across the human erythrocyte membrane is mediated by two simple asymmetric carriers operating in antiparallel fashion. Using this model and the data of zero-trans and equilibrium-exchange, it is shown that the predicted half-saturation constants for both uptake and efflux in infinite-cis conditions fully agree with the experimentally derived values. Further analysis of the kinetic data indicate that the translocation of the unloaded alpha-carrier is the rate-limiting step in galactose uptake. Under equilibrium-exchange conditions the unloaded carrier is asymmetrically distributed across the membrane so that its concentration is 8 times higher on the inner side of the membrane. Using the value of 3.3.10(5) hexose carriers per cell, the turnover number of galactose exchange is 6.5.10(4) molecules/carriers per min.

Biological Transport, Active↗
PubMed1980

Effects of anesthetic alcohols on membrane transport processes in human erythrocytes.

1. Anesthetic alcohols (pentanol, hexanol and heptanol) were found to increase the fluidity of red cell membrane lipids as monitored by the fluorescence depolarization of diphenylhexatriene. The relative potency of the alcohols was found to be parallel to their relative membrane/water partition coefficients. 2. Hexanol had biphasic effect on erythritol uptake by simple diffusion by red cells. At concentrations less than 9 mM, there was an approximately linear increase in erythritol permeability with increasing alcohol concentration. 3. The facilitated transport of uridine was markedly inhibited by hexanol. Hexanol at 6 mM produced a 65% inhibition of uridine (4 mM) uptake. Hexanol decreased both the apparent Km and V values for the equilibrium exchange of uridine. 4. The facilitated transport of galactose was only slightly inhibited by hexanol. 5. Hexanol was without effect on the passive and active fluxes of Na+ and K+ in red cells with altered cation contents. Cells that were slightly depleted of K+ and cells that were highly K+ -depleted were both insensitive to hexanol.

Alcohols↗
PubMed1983

Kinetic tests of models for sugar transport in human erythrocytes and a comparison of fresh and cold-stored cells.

We studied the time course of the entry of galactose into human erythrocytes from an external concentration of 500 mM, and analyzed the data by an integrated rate equation treatment. We found evidence for only a single, high-affinity site for sugar at the inner face of the membrane. We studied the effect of pre-loading cells with galactose at various concentrations on the entrance of galactose into the cell from 128 mM, and compared the result we found with a previous report of a similar experiment from 500 mM external sugar. We found no evidence of other than a high affinity for sugar at the inner face of the membrane. The data reject a model in which sugar transport occurs on two asymmetric, oppositely directed carriers. We studied exchange of glucose into and out of the cells as a function of sugar concentration, taking care to minimize metabolism of sugar. We found no evidence for other than a single component for glucose exchange. Our data reject the 'allosteric pore' model for sugar transport. The explanation of the high-affinity site for sugar at the inner membrane face thus remains enigmatic. We find a very significant difference in the kinetics of glucose exchange when we compare freshly drawn and long cold-stored blood. The Km for exchange was almost twice as large for cold-stored as for fresh blood.

3-O-Methylglucose↗