Date of Award

1987

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Molecular and Cellular Biology and Pathobiology

College

College of Graduate Studies

First Advisor

Lewis Stillway

Second Advisor

Donald A. Sens

Third Advisor

G. E. Lindenmayer

Fourth Advisor

Rosalie Crouch

Abstract

Catabolism of the essential branched-chain amino acids (BCAA) by skeletal muscle of the rat was investigated. In diaphragm muscles incubated in vitro oxidation and transamination of leucine or valine were accelerated by diabetes or starvation overnight. In the presence of pyruvate, transamination of BCAA by diaphragms was increased; oxidation of BCAA by diaphragms from diabetic or starved rats was increased by pyruvate but was inhibited in diaphragms of normal, fed rats. The effects on transamination were verified by direct measurements of the alpha-ketoisocaproate (KIC) product of leucine transamination. The effect of pyruvate on KIC oxidation by diaphragms was similar to the effect on leucine oxidation. Branched-chain alpha-keto acid dehydrogenase (BCDH) catalyzes the committed step in BCAA catabolism, its activity is regulated by a reversible phosphorylation-dephosporylation cycle. A method to measure active (initial) and total (active + inactive) BCDH in a soluble preparation from muscle was developed. Initial BCDH activity in fed, postabsorptive rats was about 1 nmol/min/g muscle. Total activity was ~35 nmol/min/ g muscle; thus ~2% of BCDH was in the active state. Intravenous infusion of leucine rapidly increased BCDH activity in a dose-dependent fashion; activity subsequently declined in parallel with plasma leucine concentration. Valine had no effect on BCDH activity while isoleucine was effective only at high doses. BCDH activity increased after 25% or 50% but not 9% protein meals in rats fed an adequate protein diet. Feeding a 50% protein diet chronically increased postabsorptive BCDH. Dose-dependent response of BCDH to leucine was blunted in rats fed 9% protein chronically. In insulinopenic diabetic rats plasma BCAA and muscle BCDH activity were increased. Insulin therapy or adrenalectomy of diabetic rats decreased plasma BCAA but BCDH activity remained elevated. Starvation increased BCDH activity significantly only after four days. BCDH activation in response to increased leucine was blunted in starved or diabetic rats. Conclusions: In normal fed rats, muscle BCDH activity increases in response to hyperleucinemia from meal consumption or infusion. The response to hyperleucinemia is attenuated by diabetes, starvation or consumption of a low protein diet. In diabetes increased muscle BCAA oxidation results from increased BCAA transamination and increased BCDH activity.

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