Characterization of sterol carrier protein binding with 7-dehydrocholesterol and vitamin D3.

Abstract

The interaction of rat liver sterol carrier protein (SCP) and 7-dehydrocholesterol or vitamin D3 was analyzed by the method of Scatchard plots and gel filtration on Sephadex G-100. Scatchard plots of binding data revealed that binding behavior for 7-dehydrocholesterol was monophasic, while that for vitamin D3 was biphasic. In 7-dehydrocholesterol, the apparent number of binding sites and apparent association constant K were 0.72 nmol/mg protein and 8.75 .times. 10-7 M-1, respectively. Vitamin D3 showed 2 types of binding sites and K for high affinity binding were 0.65 nmol/mg protein and 7.08 .times. 10-7 M-1, those for low affinity binding were 1.51 nmol/mg protein and 0.36 .times. 10-7 M-1, respectively. Gel filtration of SCP on Sephadex G-100 column gave 3 protein peaks (peaks I, II and III protein according to the elution orders; Ve/Vo [elution volume/void volume] = 1.0, 1.56 and 2.29, respectively). 7-Dehydrocholesterol bound with peak III protein, while vitamin D3 bound with peaks II and III protein, respectively. The MW of peak II protein was estimated to be 44,000, and that of peak III protein was 16,000. SCP apparently was involved in transformation of 7-dehydrocholesterol to cholesterol and could bind vitamin D3.