A 52-year-old man with a 20-year history of heavy alcohol use presents for routine follow-up. He reports continued drinking and right upper quadrant discomfort. Examination reveals hepatomegaly and spider angiomata. Laboratory studies: AST 180 IU/L, ALT 75 IU/L, GGT 310 IU/L, MCV 102 fL. Hepatitis B and C serologies are negative.
Which of the following best explains why measured ALT activity is lower than expected for the degree of hepatocellular injury in this patient?