Ion dependence of cystine and lysine uptake by rat renal brush-border membrane vesicles.

The shared transport system for uptake of L-cystine and L-lysine was examined in isolated rat renal brush-border membrane vesicles for the ionic requirements for activation of the system. No requirement for sodium was seen for either cystine or lysine influx. However, the efflux of lysine from the vesicle was stimulated by Na+. Therefore, the transport system appears to be asymmetric in its requirement for sodium. Two different divalent cations were used in the membrane isolations which resulted in different responses of cystine uptake to the electrogenic movement of K+ out of the vesicle. Membranes prepared by Mg-aggregation showed no stimulation of cystine influx by the imposition of a transient interior negative potential while vesicles prepared by Ca-aggregation did respond to electrogenic stimulation by an outwardly directed K-diffusion potential in the presence of valinomycin. Lysine influx was stimulated by electrogenic potassium efflux in both Mg-prepared and Ca-prepared membranes. No difference in sodium requirement for cystine influx was seen between the vesicles isolated by different cation-aggregation methods.     

Dietary cystine and liver triacylglycerols in rats: effects of dietary lysine and threonine

Addition of excess cystine to a wheat gluten diet did not alter rat liver triacylglycerols or serum cholesterol. However, if the cystine-enriched diet was supplemented with lysine and threonine, rats accumulate triacylglycerols and show increased serum cholesterol. Increases in hepatic triacylglycerols can be prevented by the further addition of methionine. This diet further increases serum cholesterol. We conclude that accumulation of triacylglycerols in the liver might be due to an increased methionine requirement, induced by the addition of excess cystine, and therefore to choline deficiency.     

Glycosphingolipids of normal and leukemic human leukocytes

Summary We have reviewed the studies on neutral glycosphingolipids and gangliosides of normal and leukemia human leukocytes. In this review, we examine (a) the glycosphingolipid composition of various leukocyte populations, (b) the differences in glycosphingolipids found among subsets of these cells, (c) the possible use of these compounds as markers of differentiation, and (d) the changes in glycosphingolipid composition that occur with leukemogenesis.     

pH-profile of cystine and glutamate transport in normal and cystinotic human fibroblasts

In the human recessive condition cystinosis, cystine transport has been reported to be normal in the plasma membrane but defective in the lysosome membrane. A possible explanation is that the transport systems at the two cellular sites are identical and that the defect in cystinosis affects the porter's ability to operate at the low pH of the lysosome. To test this hypothesis the uptake of 3H-labelled cystine and glutamate by normal and cystinotic human skin fibroblasts has been measured in vitro at pH 5.8, 6.5, 7.0, 7.4 and 8.0. Uptake of glutamate was more rapid than that of cystine. Uptake of cystine increased with increasing pH, but uptake of glutamate showed no marked pH-dependence. Transport in cystinotic cells was similar to that in normal cells, and similarly affected by pH. This finding is incompatible with the hypothesis proposed above. It is concluded that the cystine porters of the plasma membrane and the lysosome membrane are probably genetically distinct.     

Circadian variation in cell-adhesion molecule expression by normal human leukocytes

Adhesion molecules located on the surface of blood-borne leukocytes permit adherence of leukocytes to the microvascular endothelium, diapedesis of leukocytes across vessel walls, formation of intimate multicell interactions, and enhanced transmembrane signal transduction. Since some leukocyte-mediated immune functions exhibit nocturnal intensification, the current study was conducted to investigate the hypothesis that expression of selected cell adhesion molecules (CAM) varies with circadian periodicity. Blood was collected from normal human donors over a 24-h period and CAM expression by monocytes, neutrophils, and lymphocytes evaluated by monoclonal antibody binding and flow cytometry. All leukocyte classes exhibited significant circadian-like variation (p < 0.05) in CD62L (L-selectin) expression. Similarly, a diurnal variation (p < 0.05) in monocyte and neutrophil CD54 (ICAM-1) was observed. Finally, neutrophils demonstrated a circadian-like variation (p < 0.05) in CD11a (LFA-1a). The rhythmic alterations in CAM expression may be clinically relevant, since changes in CAM expression have the potential to modulate the leukocyte-induced pathogenesis associated with disease progressions such as nocturnal asthma, the nighttime exacerbations of rheumatoid arthritis, and the high nocturnal incidence of cerebrovascular and cardiovascular crisis.