Why human cytochrome P450c21 is a progesterone 21-hydroxylase.

Human cytochrome P450c21 (steroid 21-hydroxylase, CYP21A2) catalyzes the 21-hydroxylation of progesterone (P4) and its preferred substrate 17α-hydroxyprogestrone (17OHP4). CYP21A2 activities, which are required for cortisol and aldosterone biosynthesis, involve the formation of energetically disfavored primary carbon radicals. Therefore, we hypothesized that the binding of P4 and 17OHP4 to CYP21A2 restricts access of the reactive heme-oxygen complex to the C-21 hydrogen atoms, suppressing oxygenation at kinetically more favorable sites such as C-17 and C-16, which are both hydroxylated by cytochrome P450c17 (CYP17A1).

Characterization of nuclear betaII-tubulin in tumor cells: a possible novel target for taxol.

As the subunits of microtubules, alpha- and beta-tubulins have been thought to only exist in the cytoplasm where they are incorporated into microtubules. However, the beta(II) isotype of tubulin has recently been observed in the nuclei of rat kidney mesangial cells [Walss et al., 1999: Cell Motil.

Occurrence of nuclear beta(II)-tubulin in cultured cells.

Microtubules are cylindrical organelles that play critical roles in cell division. Their subunit protein, tubulin, is a target for various antitumor drugs. Tubulin exists as various forms, known as isotypes.