Minimum structural requirements for minoxidil inhibition of lysyl hydroxylase in cultured fibroblasts

The structural features of minoxidil contributing to its inhibitory effect on lysyl hydroxylase in cultured fibroblasts were investigated. Minoxidil analogs in which the pyrimidine ring (two nitrogens) was replaced with a pyridine ring (one nitrogen) or a sym-triazine ring (three nitrogens) suppressed lysyl hydroxylase activity without affecting prolyl hydroxylase activity, as did an analog in which the piperidine ring was replaced with an N,N-diethyl group. By contrast, minoxidil analogs bearing an N-monoalkyl (ethyl or butyl) group in place of the piperidine substituent failed to suppress lysyl hydroxylase activity. The results indicate that nitroxides of pyridine and triazine, in addition to pyrimidine, having an ortho amino group can act as specific inhibitors of lysyl hydroxylase in the cell. The minimum structural requirement for inhibitory activity appears to be an organic moiety containing a tertiary nitrogen para to the nitroxide oxygen, a condition that is best fulfilled by the piperidine ring in minoxidil. Hydroxylation of minoxidil at the 3 or 4 position of the piperidine ring had no impact on its ability to inhibit the post-translational hydroxylation of lysine during collagen biosynthesis. Fibroblasts treated with minoxidil, 3'-hydroxyminoxidil, or 4'-hydroxyminoxidil synthesized a collagen specifically deficient in hydroxylysine by approximately 70%, which completely accounted for the diminished lysyl hydroxylase activity. The percentage of total proteins synthesized as collagen was reduced minimally by minoxidil but not by 3'- or 4'-hydroxyminoxidil. The studies offer a potential means for therapeutic intervention of excessive collagen deposition during fibrosis, using minoxidil or preferably its hydroxy derivatives to limit the supply of hydroxylysine for collagen crosslink formation.