Amino acid control of proteolysis in perfused livers of synchronously fed rats. Mechanism and specificity of alanine co-regulation.

The primary control of autophagically mediated proteolysis in perfused rat liver is carried out via two alternate mechanisms in response to specific regulatory amino acids. One (L) elicits direct inhibition at low and high plasma levels, but requires a co-regulatory amino acid to express inhibition at normal concentrations. The second (H) is ineffective at normal levels and below, but active at higher concentrations. Because regulation is subject to unpredictable variability with ad libitum feeding, we have utilized rats synchronously fed 4 h day-1 to stabilize responses. Proteolytic control is seen to evolve in stages: H appears 12 h after the start of feeding; by 18 h L emerges, alternating with H in a statistically predictable way; with omission of the 24-h feeding, H disappears and L remains constant through 42 h. In both 18- and 42-h rats, alanine, glutamate, and aspartate exhibit similar inhibitory activity when added singly to the regulatory group at normal plasma concentrations. However, since alanine, but not glutamate or aspartate, evokes proteolytic acceleration when it is deleted from a full plasma mixture, alanine appears to be the sole co-regulator. Alanine yields co-regulatory effects with normal plasma leucine (0.2 mM) in 18- and 42-h animals and interacts synergistically with 0.8 mM leucine in 42-h but not in 18-h rats where leucine alone inhibits strongly. Because the inactivation of alanine amino-transferase by aminooxyacetate (determined from the conversion of [C-14]alanine to glucose) does not alter the co-regulatory and synergistic effects of alanine, regulation by alanine must be mediated from a site of recognition before transamination.