ETHANOL ADMINISTRATION ALTERS THE PROTEOLYTIC ACTIVITY OF HEPATIC LYSOSOMES

Protein accumulation in liver cells contributes to alcohol-induced hep atomegaly and is the result of an ethanol-elicited deceleration of pro tein catabolism (Alcohol Clin Exp Res 13:49, 1989). Because lysosomes are active in the degradation of most hepatic proteins, the present st udies were conducted to determine whether ethanol administration alter ed the proteolytic activities of partially purified hepatic lysosomes. Rats were fed liquid diets containing either ethanol (36% of calories ) or isocaloric maltodextrin for periods of 2-34 days. Prior to death, all animals were injected with [H-3]leucine to label hepatic proteins . Rats subjected to even brief periods of ethanol feeding (2-8 days) e xhibited significant hepatomegaly and hepatic protein accumulation com pared with pair-fed control animals. Crude liver homogenates and isola ted lysosomal-mitochondrial and cytosolic subfractions were incubated at 37 degrees C, and the acid-soluble radioactivity generated during i ncubation was measured as an index of proteolysis. At neutral pH, in v itro protein breakdown in incubated liver homogenates and subcellular fractions from control and ethanol-fed rats did not differ significant ly. The extent of protein hydrolysis increased when samples were incub ated at pH 5.5, which approximates the pH optimum for catalysis by lys osomal acid proteases. Under the latter conditions, partially purified lysosomes from control animals had 5-fold higher levels of proteolysi s than corresponding fractions from ethanol-fed rats. The difference i n proteolytic capacity appeared to be related to a lower latency and a higher degree of fragility of lysosomes from ethanol-fed rats at the acidic pH. The results suggest that ethanol induced alterations in lys osomal membranes may be partially responsible for their altered capaci ties for protein hydrolysis. Such changes may result from ethanol-rela ted alterations in lipid metabolism that may affect lysosome biogenesi s or the maturation of lysosomes from autophagic vacuoles.