Abstract
During purification of CYP27A from rabbit liver mitochondria, a cytochrome P450 of different molecular size was co-isolated. The latter enzyme has an apparent Mr 51,000 which is slightly lower than that of CYP27A. The 51,000-Mr protein was found to be present in mitochondria from liver, small intestine, kidney, and spleen but not in lung, testis, heart, or brain mitochondria. Determination of the N-terminal sequence revealed that the 51,000-Mr protein is a truncated form of CYP27A lacking the first 12 residues. The truncated enzyme was less efficient than the full-length CYP27A in the 27-hydroxylation of C27-sterols and much less efficient in the 25-hydroxylation of 1α-hydroxyvitamin D3. The Km values for cholesterol and 5β-cholestane-3α,7α,12α-triol were about the same with both enzymes whereas the Km for 1α-hydroxyvitamin D3 was much higher with the truncated CYP27A. The results strongly indicate that the 51,000-Mr protein is formed via proteolytic processing of CYP27A by endogenous protease(s) in some of the tissues examined. The truncation at the N terminus markedly impairs the ability of CYP27A to use 1α-hydroxyvitamin D3 as substrate and to catalyze 25-hydroxylation in the bioactivation of vitamin D3.
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