Metabolic effects of 4-pentenoate on isolated dog kidney tubules

The effects of 4-pentenoate (0.01 to 10 mM) were studied on suspensions of cortical tubules and of thick ascending limbs (TAL) prepared from dog kidneys. When cortical tubules were incubated with 1 mM glutamine, 4-pentenoate accelerated glutamine uptake, ammoniagenesis, and the production of alpha-ketoglutarate, lactate and pyruvate, but decreased gluconeogenesis. With 5 mM glutamine, the marked accumulation of alpha-ketoglutarate reversed the net fluxes through the alanine and aspartate aminotransferases. When cortical tubules or TAL were incubated with lactate, its utilization and gluconeogenesis (in cortical tubules) were markedly decreased by 4-pentenoate. The mitochondrial NAD+/NADH ratio was markedly increased by 4-pentenoate in cortical tubules but not in TAL. The production of 14CO2 from 14C[1]-pyruvate or 14C-[1]-alpha-ketoglutarate was decreased by approximately 60% by 4-pentenoate in cortical tubules but not in TAL. In cortical tubules, these findings are best explained by depletion of mitochondrial free CoA, inhibition of pyruvate and alpha-ketoglutarate dehydrogenases and decreased mitochondrial NADH. By contrast, in TAL, accumulation of reducing equivalents probably resulted from the metabolism of 4-pentenoate itself.

Experimental Fanconi's syndrome resulting from 4-pentenoate infusion in the dog

Studies were performed in anesthetized dogs to evaluate the effects of 4-pentenoate on urinary electrolyte excretion and renal metabolism. The intravenous administration of 4-pentenoate (1 mumol.kg-1.min-1 during 180 min) markedly increased the urinary excretion of bicarbonate, phosphate, potassium, amino acids, glucose, and various organic anions, whereas that of sodium and chloride also rose but less strikingly. These results suggest that 4-pentenoate markedly inhibits the proximal reabsorption of various solutes and therefore reproduces an experimental Fanconi's syndrome. Despite the rise in renal cortical concentration of alpha-ketoglutarate, glutamine utilization and total ammonia production expressed per 100 ml glomerular filtration rate increased following 4-pentenoate infusion, the ammonia being diverted into the renal vein. This increment in glutamine utilization was equal to the combined rise in the renal production of glutamate and alpha-ketoglutarate. By contrast, renal lactate utilization was drastically reduced. A causal relationship between the decreased renal cortical ATP concentration and the inhibited reabsorption of various solutes is suggested but cannot be established unequivocally.

Enzymatic basis of renal adaptation to acidosis in the dog

The effect of pH on the activities of various enzymes along the ammoniagenic pathway was tested on dog kidney cortex homogenates in an attempt to identify the metabolic steps that could be directly influenced by a low pH in this species. The activity of alphaketoglutarate dehydrogenase was markedly stimulated by acidification of the medium that decreased abruptly the km for alphaketoglutarate. By contrast, succinyl CoA synthetase and malate dehydrogenase remained relatively insensitive to pH changes. The apparent km of malic enzyme for malate was markedly decreased by an acid medium. Therefore these findings suggest that an acid pH regulates the ammoniagenic pathway at two critical sites: alphaketoglutarate dehydrogenase and malic enzyme. These stimulated enzymatic activities may account for the changes in renal cortical concentrations of metabolites observed during metabolic or respiratory acidosis.

Selective externalization of an ATP-binding protein structurally related to the clathrin-uncoating ATPase/heat shock protein in vesicles containing terminal transferrin receptors during reticulocyte maturation

Transferrin receptors are lost from reticulocytes in vesicles that are released during the final stage of erythrocyte maturation (Pan, B. T., and Johnstone, R. M. (1983) Cell 33, 967-977). Transferrin receptor-containing vesicles have a major protein component present in a 1:1 ratio with the receptor that migrates on sodium dodecyl sulfate gels as two polypeptides of Mr = 71,000 and 72,000. The Mr = 71,000/72,000 doublet is indistinguishable from the clathrin-uncoating ATPase/heat shock protein based on cross-reaction with affinity-purified antibody against the uncoating protein, by comparison of peptide maps of the Mr = 72,000 and 71,000 polypeptides and the uncoating protein, and by selective binding of these polypeptides to ATP-agarose. This finding suggests a possible activity of proteins related to the uncoating/heat shock protein family in the disposal of aged membrane proteins by a pathway independent of lysosomes.