Isotope kinetics of human skin cholesterol secretion

Specific radioactivity (SA) time curves of plasma and skin surface cholesterol collected at several skin areas were recorded in 10 patients on formula diets after single intravenous injections of radioactive cholesterol. Earliest detectable radioactivity on skin surface was found in 4-6 days; depending on the skin site, SA's peaked in 13-75 days. SA's of free cholesterol were almost always higher than those of esterified cholesterol. The general forms of the SA time curves were in keeping with the idea that plasma cholesterol is carried to the skin surface in association with the epidermal and sebaceous cells, whereby (a) cholesterol synthesized de novo is mixed with derived from plasma and (b) appearances of plasma cholesterol on the skin surface is delayed by a time factor that corresponds to the movement of epidermal and sebaceous cells from the basal layer to the skin surface. The shorter mean transit times of plasma cholesterol on skin areas rich in sebaceous glands (22-24 days on the head) than on those poor in these glands (38 days on forearms and 72 days on feet) suggest that cholesterol passes faster through the sebaceous glands than through the epidermis, and faster through thin than thick epidermis. The fraction of skin surface cholesterol (f) that is derived from plasma cholesterol was estimated by three independent methods, and comparable results were obtained. Values of f were lower on skin areas rich in sebaceous glands (0.29-0.46 on forehead) than on areas poor in these glands (0.41-0.70 for forearms; 0.60 on feet) and lower for esterified (0.27-0.33) than for free (0.39-0.48) cholesterol. These data suggest that higher proportions of sebaceous gland and of esterified cholesterol, respectively, are synthesized de novo than epidermal and of free cholesterol. In two patients it was possible to calculate that f of total skin surface cholesterol was 0.49 and 0.37, respectively, and that the maximum amount of plasma cholesterol lost through the skin was 29 and 22 mg/day, respectively. Knowing the total daily excretion of total neutral and acidic steroids in feces in these patients, and assuming a total daily urinary steroid excretion 50 mg, we estimated that no more than 3.2% of total steroid excretion occurred via the skin.