ABSTRACT
Obtaining information on the dynamics of vitamin A turnover requires studies that are not generally feasible in humans. For example, the kinetic studies referred to above involved the intravenous administration of in vivoprepared radiolabeled vitamin A to substantial numbers of normal rats and the collection of serial blood samples for 35-115 days. In addition, tissue vitamin A tracer and tracee dynamics were determined. For obvious reasons, comparable studies cannot currently be carried out in humans. recently some unpublished and unique data on plasma retinol turnover in humans became available (D. S. Goodman, personal communication, 1990). In work done in 1965-1966 to study retinol-binding protein, three adult male subjects received an intravenous injection of 15-[14C]retinol in autologous plasma. For each subject, the labeled dose was prepared by incubating a solution of the isotope in 0.5 ml ethanol wHh 10 ml of freshly obtained plasma. After several hours of incubation at 37°C and at room temperature, the plasma was sterilized filtration before administration. Blood samples (n = -20) were collected between -20 hr and 115 days (subject 1) or 240 days (subject 3). Aliquots of plasma were extracted and plasma 14C was determined. Most of the lipid extract was chromatographed on alumina columns and it was found that essentially all plasma radioactivity was in retinol throughout the Retinol masses were measured fluorimetrically.
