Vitamin D uptake and metabolism by perfused rat liver: influences of carrier proteins

Several blood proteins have been associated with the transport of vitamin D sterols. Vitamin D is synthesized in the skin or absorbed from the intestine, and there is evidence for different rates of transfer to the liver from these sources. To evaluate the influences of various plasma proteins on the hepatic accumulation of vitamin D3, human plasma albumin, D-binding protein, chylomicrons, chylomicron remnants, low density lipoprotein (LDL) and high density lipoprotein were isolated and incubated with [3H]vitamin D3 before single-pass perfusions of the isolated, vitamin D-deficient rat liver. Hepatic uptake of sterol was greatest when vitamin D3 was presented on LDL or chylomicron remnants, whereas D-binding protein permitted the least uptake. Silicic acid chromatography revealed greater amounts of 25-hydroxyvitamin D3 when substrate was presented on carriers known to have hepatic receptors. After iv administration of tracer amounts of vitamin D3 to fasting rats, gradient gel electrophoretic analyses of plasma revealed most of the [3H] associated with the vitamin D-binding protein, and smaller amounts associated with LDL and high density lipoprotein. Our results suggest a major role for chylomicron remnants in the hepatic presentation of ingested vitamin D3 and support the possibility that hepatic delivery from cutaneous sites may involve lipoprotein carriers.