Hypercatabolism of lipoprotein-free apolipoprotein A-I in HDL-deficient mutant chickens.

The Wisconsin Hypoalpha Mutant (WHAM) chicken has a sex-linked mutation associated with a 90% reduction in high-density lipoprotein (HDL) cholesterol and apolipoprotein A-I (apoA-I). In the present studies, we did not detect a defect in apoA-I synthesis or secretion in liver or intestine. We tested the hypothesis that apoA-I is not binding properly to lipoprotein particles and is undergoing hypercatabolism. We therefore studied the in vivo turnover of lipid-free 125I-apoA-I. Its turnover was fourfold faster in WHAM chickens than in normal chickens. The 125I-apoA-I equilibrated more slowly with HDL in the WHAM chickens, and these animals had a much larger steady-state pool of lipid-free apoA-I than did control chickens. To determine the tissue sites of degradation of apoA- I, the tissue distribution of 125I-tyramine cellobiose apoA-I was assessed. The liver and kidneys were the major sites of apoA-I degradation, but in the WHAM chickens, the kidney made a twofold larger contribution to apoA-I degradation than in normal chickens. Total plasma phospholipid levels are reduced by 44% to 78% in the WHAM chickens. A phospholipid deficit might explain the elevated lipid-free apoA-I pool and, secondarily, the HDL deficiency of the WHAM chickens.