TRANSPORT OF NEUTRAL AND DIBASIC AMINO ACIDS BY HUMAN LEUKOCYTES: ABSENCE OF DEFECT IN CYSTINURIA

DNA and other strands: the making of a human geneticist

This article--a mini-memoir--focuses on the first half of my half-century-long career as a human geneticist: its accidental beginnings; its early bad and then good fortunes at the National Institutes of Health; its serendipitous successes and career-making scientific productivity at Yale; and its incalculable fortuity in the form of the large number of talented and resourceful mentors, colleagues, postdoctoral fellows, graduate students, and technicians who worked with me. These years acted as a launchpad for positions of visibility and leadership that followed them. My personal odyssey, which began in Madison, Wisconsin, and meandered with no fixed plan to New York, Bethesda, New Haven, and Princeton, has offered me life views as a human and medical geneticist that are panoramic, splendid, and indelible. I doubt that many people have been as fortunate as I have been in the professional life I have lived--and continue to live.

The inhibitory effect of glutamate on the growth of a murine hybridoma is caused by competitive inhibition of the x(-) (C) transport system required for cystine utilization

Glutamic acid was found to be growth inhibitory to a murinelymphocyte hybridoma in a concentration-dependent manner from 3to 12 mM glutamate. At 12 mM glutamate there was a 70% decreasein the specific growth rate of the cells. Attempts to alleviateinhibition or adapt cells to growth in glutamate-based mediawere unsuccessful. It is proposed that elevated glutamate levelsimpair adequate uptake of cystine, a critical amino acid for thesynthesis of glutathione. Glutathione is required by cells toprevent intracellular oxidative stress. The measured rate ofuptake of U-(14)C L-cystine into the cells was found to havethe following parameters: K(m) = 0.87 mM, V(max) = 0.9nmole/mg cell protein per min. The uptake was sodiumindependent and resembled the previously described x(-) (c)transport system, with elevated glutamate levels causingextensive inhibition. Glutamate at a concentration of 1.4 mMcaused a 50% decrease in cystine uptake from the serum-freegrowth medium. Glutamate was taken up from the external medium(K(m) = 20 mM and V(max) = 12.5 nmole/mg cell protein permin) by the same transport system in a stereo specific, sodiumindependent manner. Of the amino acids examined, it was foundthat cystine and homocysteic acid were the most extensiveinhibitors of glutamate uptake and that inhibition was competitive. Metabolic profiles of the cells grown in culturescontaining enhanced glutamate levels revealed an overallincrease in net production of alanine, serine, asparagine andaspartate. A substantially increased specific consumption ofglutamate was accompanied by a decreased consumption of cystine,valine and phenylalanine.The combined kinetic and metabolic results indicate thatglutamate and cystine are taken up by the anionic transportsystem x(-) (c). The increasing levels of glutamate in themedium result in a decreased transport of cystine by this systemdue to competitive inhibition by glutamate.

Changes in neutral amino acid transport activity in myeloid leukemia cells differentiated by lipopolysaccharide

M1 cells derived from mouse myeloid leukemia have been reported to differentiate to macrophage-like cells upon treatment with substances such as lipopolysaccharide. Previously we found that in mouse peritoneal macrophages most of the neutral amino acids were taken up through a unique Na+-independent system. In this paper we have investigated the neutral amino acid transport in M1 cells and in those treated with lipopolysaccharide. In M1 cells serine, alanine and proline were taken up mainly by Na+-dependent transport systems, and leucine was largely transported by a Na+-independent system. By treating the cells with lipopolysaccharide, the activities of the Na+-dependent systems markedly decreased, whereas the activity of the Na+-independent system was little affected. The amino acid concentrations in the cells and the culture medium were measured. As a whole, the intracellular to extracellular distribution ratios for neutral amino acids that are preferred substrates for Na+-dependent systems were decreased on lipopolysaccharide treatment, whereas those for amino acids that are mainly transported by a Na+-independent system were slightly increased. From these results we conclude that M1 cells treated with lipopolysaccharide tend to differentiate to macrophage-like cells with respect to the neutral amino acid transport.

Cystinuria

Cystinuria is an inherited metabolic disease resulting in renal stone formation. An incidence of 1 in 7000 makes it a relatively common genetic disease. The biochemical defect is a carrier protein in the epithelial cells of certain organs. This carrier protein is responsible for the transport of cystine and the dibasic amino acids. Cystine is a poorly soluble compound which precipitates in acid urine and results in renal calculi. Cystine stones account for 1 to 2% of all renal calculi. Homozygotes are detected by the high concentration of cystine in their urine. Treatment consists of sulfhydryl compounds that form more soluble compounds with cystine through sulfhydryl exchange as well as alkalinization of urine and hydration to make cystine more soluble.

Intracellular amino acid levels as predictors of protein synthesis

Protein synthesis depends on a complete complement of precursor amino acids, specific acetylating enzymes, tRNA, and so forth. It has been related to metabolism of individual amino acids, eg, valine and leucine; however, the relation of protein synthesis to the ambient concentrations of amino acids in the intracellular and extracellular pools has not been defined. Using the viable, isolated granulocyte (leukocyte) as an in vitro cell model, protein synthesis (incorporation of 4,5-3H-leucine) has been related to simultaneous amino acid concentrations in the cell and plasma by multiple regression analysis. Fifty-five normal neonates and 30 normal adults were studied. Protein synthesis was higher in the infants than in the adults (3,527 vs 2,685 pmole/hr/mg DNA). The intracellular concentrations of most amino acids were higher than their concentrations in plasma, except for valine and citrulline, which were lower. The "aminograms" in the two pools also were very different. Forty-four percent of the variance (R2) in protein synthesis was accounted for by the intracellular concentrations of leucine, glycine, alanine, and taurine in neonates and 45% by a combination of threonine, valine, methionine, and histidine in adults. The intracellular concentrations of each of these predictor amino acids in adults were, in turn, related to different combinations of the plasma concentrations of threonine, phenylalanine, tryptophan, isoleucine, histidine, citrulline, ornithine, arginine, and glycine. Thus, it is possible to identify sets of intracellular amino acids that predict the level of protein synthesis and to delineate combinations of plasma amino acids whose levels account for a significant portion of the variance in the intracellular predictor amino acids in normal human infants and adults.

Uptake of L-[35S]cystine by isolated rat brain capillaries

Adult rat brain capillaries were isolated by a simplified procedure and showed an enrichment of the marker enzyme, gamma-glutamyltranspeptidase. The uptake of [35S]cystine at 37 degrees C by this preparation can be divided into two components, a sodium- and energy-dependent transport process for the free amino acid pool, with an apparent Km of 36 microM, and a binding process, with an apparent Km of 1.13 mM. Chemical analysis of the amino acid pool indicates that cystine is the major from of intracapillary 35S. Cystine transport was not inhibited by lysine, but glycine, alpha-methylaminoisobutyric acid and beta-2-aminobicyclo-[2,2,1]-heptane-2-carboxylic acid were inhibitory to a small extent.

Developmental and other characteristics of lysine uptake by rat brain synaptosomes

Synaptosomes isolated from adult or newborn rat cerebrum take up L-lysine by two saturable systems, one with a high affinity low capacity and the other with a low affinity high capacity. Initial rate of uptake for low lysine concentrations is mort tissue. Analysis of kinetic data indicates that synaptosomes of the newborn have a higher Vmax than those of the adult for high affinity system but adult for high affinity system but adult synaptosomes have a higher Vmax than newborn for low affinity system. At a physiological lysine concentration of 0.5 mM, the calculated contributions of two systems indicate that the adult uptake occurs for about 71% by low affinity system but the newborn utilizes both systems to the same extent. The uptake is sodium independent but pH dependent. Lysine uptake is inhibited by other dibasic amino acids, arginine and ornithine but not cystine. Kinetic analysis indicates that arginine specifically inhibits the high affinity, low Km system for lysine uptake.

Elucidation of an A and L system for amino acid transport in the human lymphoblast using a membrane filtration technique

Optimum conditions have been established for the measurement of amino acid transport by human lymphoblastoid cell lines using a membrane-filtration technique. The parameters we found to be important for the reproducibility of the method are: the types and combination of filters, the strength of the vacuum applied to the filters and the density of the cultures at the time of harvesting and during uptake and filtration. We found that bovine serum albumin added to phosphate buffered saline (PBS) glucose in which the cells are washed, resuspended and assayed is essential for the maintenance of viability, the prevention of clumping and the retention of the accumulated amino acid. Using this procedure we have characterized two transport systems for the neutral amino acids; an A and an L system, which are similar but not identical to the A and L systems characterized in rodent cell lines. These A and L systems have characteristically lower Km's and Vm's for alanine and phenylalanine, when compared to rodent cell lines. In addition, we find alpha-AIB to be a poor competitor of alanine and phenylalanine uptake.

Distribution of tryptophan in erythrocytes, leukocytes and thrombocytes, and its binding to plasma albumin

The extended theory about a dysfunction of the serotoninergic system in depression and schizophrenia includes the hypothesis of a disturbance in the transport systems of tryptophan and tyrosine from blood to brain. It would be interesting to know if blood cells may be used as a model for the central transport mechanisms of these amino acids. After an oral load, the in vivo distribution of L-tryptophan (50 mg/kg) was studied in the blood plasma, in the different blood cells and its binding to plasma albumin, in six healthy, seven schizophrenic and two depressive subjects. In all the compartments studied, tryptophan reached a peak, 1--2 hours after the load. Before and after the load, the variation of the tryptophan concentration in the erythrocytes was parallel to the plasma free tryptophan, whereas the uptake of this amino acid was higher in leukocytes and thrombocytes than in erythrocytes. However, this model does not show differences between schizophrenic and normal subjects with regard to the transport of tryptophan and tyrosine in these cells.

alpha-L-iduronidase activity in leukocytes: diagnosis of homozygotes and heterozygotes of the Hurler syndrome

The activity of alpha-L-iduronidase was determined in leukocytes from two patients with the Hurler syndrome, five obligatory heterozygotes, one patient with the Hunter syndrome, and ten normal individuals. It was found that the determination of alpha-L-iduronidase in leukocytes was a useful method for differential diagnosis between the Hurler and Hunter syndromes. Heterozygotes of the Hurler syndrome showed approximately 50% level of alpha-L-iduronidase activity in leukocytes as compared with that of normal individuals. This suggests that the determination of alpha-L-iduronidase activity may be available for the carrier detection of the Hurler syndrome.

Cystinuria and its relationship to mental retardation

Urine specimens from 2,073 mentally retarded children have been analysed for their amino acid content, with the aim of determing the relationship of cystinuria and mental retardation. No children homozygous for cystinuria were identified, but 50 heterozygous children were found. This heterozygote incidence is 13 times that expected in our general population.

Uptake and utilization of exogenous cystine by cystinotic and normal fibroblasts

The uptake of l-[(35)S]cystine was studied in six cystinotic and six normal fibroblast lines grown for five days either on cover slips or in 32-oz plastic flasks. Cystinotics showed greater uptake than normals. The apparent K(t) for cystine entry in both types of cells was 0.043 mM but cystinotic cells showed a higher maximum velocity of entry. A comparison of the fate of l-[(35)S]cystine incubated for 20 min with monolayers of cells showed 30% and 15% of the intracellular (35)S to be l-cystine in cystinotic and normal cells, respectively. The (35)S effluxed more slowly from cystinotic than from normal cells after a 20-min preloading with l-[(35)S]cystine. Identification of (35)S compounds in efflux media after 3 min showed 75% of the total (35)S was l-cystine with the remainder in cysteine and acidic sulfur metabolites of cystine with no essential difference between cystinotics and normals. In paired experiments, the specific activity of the effluxed l-[(35)S]cystine after both efflux periods was the same as that entering the cell, thus indicating that the free l-[(35)S]cystine had not exchanged with the pre-existing pool in the cystinotic cells. During 3 min efflux, the l-cystine pool in normal cells was depleted mainly by loss of free cystine. In cystinotic cells, a new steady state was attained after 21 min of efflux and the intracellular l-[(35)S]cystine had the same percentage of total radioactivity seen after the initial 20-min uptake. After the rapid efflux of l-[(35)S]cystine from normals, [(35)S]cysteine and other labeled cystine metabolites appeared in the efflux media. By the end of a 3-min efflux, cystinotic cells had incorporated more label into reduced glutathione than had normal cells. However, when the new steady state was attained in cystinotics, the amounts of (35)S in glutathione were not markedly different in the two types of cells. Approximately 95% of the total label could be accounted for in free sulfur compounds. The data show an increased uptake and decreased efflux of cystine from cystinotic cells. However, it is not possible to conclude if these differences are due to primary changes in membrane function or to the reflection of metabolic defects without further investigation.

Inhibition of amino acid incorporation into protein of human neutrophils by phagocytosis

The incorporation of uniformly labeled (14)C-amino acids into total protein of human polymorphonuclear leukocytes is inhibited when the cells are challenged with heat-killed Escherichia coli or latex particles. Parallel studies of amino acid incorporation and glucose oxidation indicate that the inhibition is dependent upon ingestion of the particles. It is suggested that the process of phagocytosis might inhibit the transport of amino acids into the cell either by competing for available energy or by internalizing specific transport sites.

Transport of dibasic amino acids, cystine, and tryptophan by cultured human fibroblasts: absence of a defect in cystinuria and Hartnup disease

Transport of lysine, arginine, cystine, and tryptophan was studied in cultured skin fibroblasts from normal controls and from patients with cystinuria and Hartnup disease. Each of these amino acids was accumulated against concentration gradients by energy-dependent, saturable mechanisms. Lysine and arginine were each transported by two distinct processes which they shared with each other and with ornithine. In contrast, cystine was taken up by a different transport system with no demonstrable affinity for the dibasic amino acids. The time course and Michaelis-Menten kinetics of lysine and cystine uptake by cells from three cystinuric patients differed in no way from those found in control cells. Similarly, the characteristics of tryptophan uptake by cells from a child with Hartnup disease were identical to those noted in control cells. These findings indicate that the specific transport defects observed in gut and kidney in cystinuria and Hartnup disease are not expressed in cultured human fibroblasts, thus providing additional evidence of the important role that cellular differentiation plays in the regulation of expression of the human genome.

Amino acid oxidase in leukocytes: evidence against a major role in phagocytosis

The addition of either d- or l-amino acids fails to increase hexose monophosphate shunt activity of resting or phagocytizing neutrophils. This is presumptive evidence against a major role for amino acid oxidase in the bactericidal activity of the cell.

Increased cystine in leukocytes from individuals homozygous and heterozygous for cystinosis

In patients with cystinosis, the concentration of free cystine in leukocytes was 80 times greater than normal, and six times the normal content for their parents. This is the first demonstration of an abnormality in heterozygotes for this rare inherited disease of childhood. Three-quarters of the cystine was recovered in the granular fraction of cystinotic leukocytes.