SPECIFICITY OF LEUCINE EFFECT ON PROTEIN-DEGRADATION IN PERFUSED RAT-HEART

The primary objective of the present study was to assess whether the inhibitory effect of leucine in rat hearts on protein degradation is mediated by leucine itself, α-ketoisocaproate (α-KIC) or the decarboxylated products of leucine. Protein degradation, as measured by the release of phenylalanine, was inhibited by 1 mM leucine in hearts supplied with glucose, despite low intracellular α-KIC concentration (6.3 ± 0.5 μM). The inhibition of protein degradation by leucine occurred in hearts supplied with pyruvate, a substrate that completely abolished leucine decarboxylation. Under this condition, leucine was transaminated to α-KIC. Since the transamination could not be inhibited more than 38% by 10 mM L-cycloserine, it was difficult to exclude that leucine exerted its inhibitory effect via transamination to α-KIC. A clear correlation between protein degradation and I intracellular leucine or α-KIC concentration could not be established. However, a high concentration of leucine (1 mM) or α-KIC (0.3 mM) in the perfusate inhibited protein degradation by 30 ana 20%, respectively. This suggested that both leucine and α-KIC act on the plasma membrane regulatory sites. Of the various structural analogues of leucine that were tested, only L-leucinol inhibited protein degradation. This inhibition could be attributed to a direct lysosomotropic effect of this amino alcohol. The results indicate that high concentrations of extracellular leucine and α-KIC appear to inhibit heart protein degradation in vitro. © 1994 Academic Press Limited.