Amino acids regulate kidney cell protein breakdown.

Amino acids inhibit breakdown of long-lived intracellular proteins in some but not all tissues studied. Because no information is available relating to the effect of amino acids on kidney cell proteolysis, this study was conducted with cultured proximal-like opossum kidney (OK) cells and primary cultured rabbit proximal tubular cells in which long-lived cell proteins were labeled with carbon 14-labeled valine. These cultured cells were acutely deprived of amino acids; this was followed by a 57% to 66% increase in the proteolytic rate in OK cells and a 22% rate increase in the rabbit kidney cells. In cultured OK cells incubated in serum-free minimal essential medium containing 13 amino acids, proteolysis averaged 4.62% +/- 0.28%/2 hr and increased to 7.66% +/- 0.38%/2 hr when amino acids were deleted. Each amino acid was then added alone. Leucine, phenylalanine, and lysine had significant effects in inhibiting the deprivation response by 40%, 26%, and 22%, respectively. Leucine appears to inhibit proteolysis directly and not through its metabolites, since alpha-ketoisocaproate, the leucine transamination product, was without effect. Similarly, failure of tyrosine to inhibit proteolysis suggests a direct phenylalanine action. When leucine, phenylalanine, and lysine were simultaneously deleted from the incubation medium, the increase in proteolysis corresponded to 56% of the response after deletion of all amino acids. Thus to maximally affect proteolysis, amino acids, which on their own have little effect on protein breakdown, also appear to play a role. From this study we conclude that amino acids seem to play an important and direct role in the regulation of kidney epithelial cell protein breakdown.