Novel metabolism of 1α,25-dihydroxyvitamin D3 with C24-C25 bond cleavage catalyzed by human CYP24A1

Our previous study revealed that human CYP24A1 catalyzes a remarkable metabolism consisting of both C-23 and C-24 hydroxylation pathways that used both 25(OH)D-3 and 1alpha,25(OH)(2)D-3 as substrates, while rat CYP24A1 showed extreme predominance of the C-24 over C-23 hydroxylation pathway [Sakaki, T., Sawada, N., Komai, K., Shiozawa, S., Yamada, S., Yamamoto, K., Ohyama, Y. and Inouye, K. (2000) Eur. J. Biochem. 267, 6158-6165]. In this study, by using the Escherichia coli expression system for human CYP24A1, we identified 25,26,27-trinor-23-ene-D-3 and 25,26,27-trinor-23-ene-1alpha(OH)D-3 as novel metabolites of 25(OH)D-3 and 1alpha,25(OH)(2)D-3, respectively. These metabolites appear to be closely related to the C-23 hydroxylation pathway, because human CYP24A1 produces much more of these metabolites than does rat CYP24A1. We propose that the C24-C25 bond cleavage occurs by a unique reaction mechanism including radical rearrangement. Namely, after hydrogen abstraction of the C-23 position of 1alpha,25(OH)(2)D-3, part of the substrate-radical intermediate is converted into 25,26,27-trinor-23-ene-1alpha-(OH)D-3, while a major part of them is converted into 1alpha,23,25(OH)(3)D-3. Because the C-24-C-25 bond cleavage abolishes the binding affinity of 1alpha,25(OH)D-3 for the vitamin D receptor, this reaction is quite effective for inactivation of 1alpha,25(OH)D-3.