The
cholesterol ozonolysis products secosterol-A and its aldolization product
secosterol-B were recently detected in human atherosclerotic tissues and brain specimens, and have been postulated to play pivotal roles in the pathogenesis of
atherosclerosis and
neurodegenerative diseases. We examined several oxidized
cholesterol metabolites including secosterol-A,
secosterol-B,
25-hydroxycholesterol, 5β,6β-epoxycholesterol and
7-ketocholesterol for their effects on the activities of three
nitric oxide synthases. In contrast to other oxidized metabolites, secosterol-A was found to be a potent inhibitor against the neuronal- and endothelial-type, but not the inducible-type
nitric oxide synthase, with IC(50) values of 22 ± 1 and 50 ± 5 µM, respectively. The
calmodulin-binding regions of the neuronal- and endothelial-
nitric oxide synthases contain
lysine residues which are not present in the inducible-type
nitric oxide synthase. Secosterol-A modifies
proteins through the formation of a
Schiff base with the
lysine epsilon-amino group. It is possible that secosterol-A modifies
lysine residues of constitutive
nitric oxide synthases, leading to the inhibition of enzymatic activities. As
nitric oxide is a critical signaling molecule in vascular function and in long-term potentiation, its reduced production through inhibition of constitutive
nitric oxide synthases by secosterol-A may contribute to the development of
atherosclerosis and memory impairment in particular
neurodegenerative diseases.