AJP - Endocrinology and Metabolism, Vol 254, Issue 4 E419-E428, Copyright © 1988 by American Physiological Society
Differences in renal metabolism of insulin and cytochrome c
J. Herrman, R. E. Simmons, B. H. Frank and R. Rabkin
Department of Medicine, Stanford University, California 94305.
Kidneys degrade small proteins such as cytochrome c (CYT c) by the classic
lysosomal pathway. However, because alternate routes for the transport and
degradation of protein hormones have been identified in other tissues, we
set out to determine whether extralysosomal sites might participate in the
renal degradation of insulin. First, we compared the effect of the
lysosomal inhibitor NH4Cl on insulin and CYT c degradation by isolated
perfused rat kidneys. After kidneys were loaded with radiolabeled proteins
to allow for absorption and transport to lysosomes, degradation was
measured in the presence or absence of inhibitors. Control kidneys degraded
45 +/- 1.5% of the trapped CYT c per hour, and this was inhibited 62 +/-
1.3% by NH4Cl. In contrast, 86 +/- 2.4% of the trapped insulin was degraded
per hour, and this was inhibited 26 +/- 4% by NH4Cl. Next we followed the
subcellular distribution of 125I-labeled insulin in kidneys exposed to
125I-labeled insulin in vivo or when isolated and perfused. Under both
circumstances the distribution of insulin on a linear sucrose gradient
differed from that of the lysosomal enzyme N-acetyl-beta-glucosaminidase.
In contrast, [14CH3]CYT c, injected in vivo, distributed over a density
similar to the lysosomal marker. Thus important differences exist between
the renal metabolism of CYT c, which proceeds in lysosomes, and the renal
metabolism of insulin. These include rate of degradation, sensitivity to
NH4Cl, and subcellular sites of localization. Accordingly, we suggest that
insulin degradation may occur, at least in part, in a different compartment
from the classic lysosomal site of protein degradation.
