EVIDENCE FOR TRANSFER OF VITAMIN Da FROM CHYLOMICRONS TO a-GLOBULINS*

The transport of vitamin D3 from the intestine via the intestinal lymph to the plasma compartment in rats has been studied. Rats were cannulated in the intestinal lymph duct and given an intraduodenal bolus of [la,2a3H] vitamin D3 in soybean oil. The appearance of radioactivity was negligible during the initial 2 h and peaked 4-8 h after feeding. About 90% of the recovered radioactivity was associated with the chylomicron fraction (d c 1.006 g/ml), 0.5% with the combined low and high density lipoprotein fraction(1.006 < d < 1.21 g/ml), while 9.5% was found in the protein fraction of d > 1.21 g/ml. No esterified vitamin D3 was detected, but in the lymph fraction with d > 1.21 g/ml significant amounts of the radioactivity co-chromatographed with 25-hydroxyvitamin D3. When r3H]vitamin D3-containing intestinal chylomicrons were incubated in ui tm with plasma of d > 1.006 g/ml, the amount of radioactivity transferred from the chylomicrons to this plasma fraction was zero at 4 "C while up to 80% was transferred at 37 "C during 3 h of incubation. The transfer was time-dependent, and dependent on the ratio between the amount of chylomicrons and plasma with d > 1.006 g/ml. The transfer of labeled vitamin Da from intestinal lymph to the plasma fraction with d > 1.006 g/ml was also found in vivo in functionally hepatectomized rats. Both in uitm and in uiuo, 91-95% of the transferred vitamin DI was recovered in the plasma fraction with d > 1.21 g/ml. Agarose gel electrophoresis of this fraction (d z 1.21 g/ml) indicated that 95% of the radioactivity was associated with the a-globulins. The transfer of vitamin D1 from chylomicrons to a-globulin in lymph and blood plasma might be of physiological importance for targeting of vitamin D3 absorbed from the intestine.