Analysis of the ability of vitamin D3-metabolizing cytochromes P450 to act on vitamin D3 sulfate and 25-hydroxyvitamin D3 3-sulfate

Abstract

25-Hydroxyvitamin D3 (25(OH)D3) is present in the human circulation esterified to sulfate with some studies showing that 25(OH)D3 3-sulfate levels are almost as high as unconjugated 25(OH)D3. Vitamin D3 is also present in human serum in the sulfated form as are other metabolites. Our aim was to determine whether sulfated forms of vitamin D3 and vitamin D3 metabolites can be acted on by vitamin D-metabolizing cytochromes P450 (CYPs), one of which (CYP11A1) is known to act on cholesterol sulfate. We used purified, bacterially expressed CYPs to test if they could act on the sulfated forms of their natural substrates. Purified CYP27A1 converted vitamin D3 sulfate to 25(OH)D3 3-sulfate with a catalytic efficiency (k_cat/K_m) approximately half that for the conversion of vitamin D3 to 25(OH)D3. Similarly, the rate of metabolism of vitamin D3 sulfate was half that of vitamin D3 for CYP27A1 in rat liver mitochondria. CYP2R1 which is also a vitamin D 25-hydroxylase did not act on vitamin D3 sulfate. CYP11A1 was able to convert vitamin D3 sulfate to 20(OH)D3 3-sulfate but at a considerably lower rate than for conversion of vitamin D3 to 20(OH)D3. 25(OH)D3 3-sulfate was not metabolized by the activating enzyme, CYP27B1, nor by the inactivating enzyme, CYP24A1. Thus, we conclude that 25(OH)D3 3-sulfate in the circulation may act as a pool of metabolically inactive vitamin D3 to be released by hydrolysis at times of need whereas vitamin D3 sulfate can be metabolized in a similar manner to free vitamin D3 by CYP27A1 and to a lesser degree by CYP11A1.

Keywords: 25-hydroxyvitamin D3; CYP11A1; CYP24A1; CYP27A1; CYP27B1; CYP2R1; Vitamin D3 sulfate.