Inhibition of reconstituted vitamin D3 25-hydroxylase by a protein fraction from rat liver microsomes.

A cytochrome P-450 fraction, containing vitamin D3 25-hydroxylase activity after reconstitution, was prepared from rat liver microsomes. The 25-hydroxylase activity increased considerably upon purification. A protein fraction isolated from the microsomes during the preparation markedly inhibited the reconstituted vitamin D3 25-hydroxylase activity. The inhibition was dependent on the amount of protein and completely reversed by heat treatment.