Control of hepatic proteolysis by leucine and isovaleryl-L-carnitine through a common locus. Evidence for a possible mechanism of recognition at the plasma membrane

• Published in: The Journal of biological chemistry Vol. 267; no. 31; pp. 22066 - 22072
• Main Authors: MIOTTO, G, VENERANDO, R, KHURANA, K. K, SILIPRANDI, N, MORTIMORE, G. E
• Format: Journal Article
• Language: English
• Published: Bethesda, MD American Society for Biochemistry and Molecular Biology 05-11-1992
• Subjects: Alanine - metabolism; Animals; Biological and medical sciences; Biological Transport; Carnitine - analogs & derivatives; Carnitine - metabolism; Cell Membrane - metabolism; Cell metabolism, cell oxidation; Cell physiology; Fundamental and applied biological sciences. Psychology; Glutamine - metabolism; Kinetics; Leucine - metabolism; Liver - metabolism; Molecular and cellular biology; Proteins - metabolism; Rats; Site specificity; Vertebrata; Mammalia; Rat; Proteolysis; Liver; Isovaleric acid; Rodentia; Plasma membrane; Inhibition; Leucine; Carnitine
• ISSN: 0021-9258; 1083-351X
• DOI: 10.1016/S0021-9258(18)41636-5

Deprivation-induced proteolysis in the perfused rat liver is controlled through the multiphasic action of 7 regulatory amino
acids of which L-leucine plays the dominant role. Recently, isovaleryl-L-carnitine (IVC) was shown to mimic the leucine's
effects, suggesting that the two molecules share structural features that are recognized at a common site(s). In this study
we find that each evokes identical responses consisting of inhibitory effects at 0.08 and 0.8 mM, separated by a sharp zonal
loss of inhibition at 0.15 mM. As monitored by density shifts of beta-hexosaminidase in colloidal silica gradients, macroautophagy
is suppressed by both. Responses to Leu and IVC at 0.08 and 0.15 mM are stereospecific and require a reactive group at the
alpha-carbon (or equivalent) and a high degree of branched chain specificity. In addition, 0.5 mM Ala coregulates with IVC
and Leu by decreasing the zonal loss at 0.15 mM. The fact that the multiphasic responses can be duplicated with equimolar
mixtures of Leu + IVC indicates that both react at the same site(s). IVC is readily taken up by a saturable process, but owing
to its rapid hydrolysis in the cell, the ratio of internal to external IVC remains low over a 4-fold concentration range.
These findings, together with a kinetic analysis of concerted responses to regulatory amino acids, suggest that the recognition
sites are at a position in the cell, possibly at the plasma membrane, to react reversibly with plasma amino acids.