Melanin potentiates gentamicin-induced inhibition of collagen biosynthesis in human skin fibroblasts.

One of the recognized side effects of gentamicin is ototoxicity. The mechanism underlying the organ specificity of this side effect of gentamicin has not been fully established. In view of the fact that a number of pharmacologic agents are known to form complexes with melanin and melanin is an abundant constituent of the inner ear tissues, we determined whether gentamicin interacts with melanin and how this process affects the biosynthesis of collagen in cultured human skin fibroblasts. Our results indicate that gentamicin forms complexes with melanin. The amount of gentamicin bound to melanin increases with increasing of initial drug concentration. The Scatchard plot analysis of drug binding to melanin showed that at least two classes of independent binding sites are implicated in gentamicin-melanin complex formation: one class with an association constant K(1) approximately 4 x 10(3) M(-1), and the second class with an association constant K(2) approximately 3 x 10(2) M(-1). The number of total binding sites (n(1)+n(2)) was calculated as about 1.36 micromol gentamicin per 1 mg melanin. We have suggested that prolidase, an enzyme involved in collagen metabolism, may be one of the targets for gentamicin-induced inhibition of collagen biosynthesis. We found that gentamicin-induced inhibition of prolidase activity (IC(50) approximately 100 microM) and collagen biosynthesis (IC(50) approximately 100 microM). At this concentration of gentamicin, DNA biosynthesis in human skin fibroblasts was inhibited only by about 30%. Melanin at 100 microg/ml produced about 25% inhibition of DNA synthesis and about 30% inhibition of prolidase activity, but it had no effect on collagen biosynthesis in cultured fibroblasts. However, the addition of melanin (100 microg/ml) to gentamicin-treated cells (100 microM) augmented the inhibitory action of gentamicin on collagen and DNA biosynthesis and partially reversed its inhibitory effect on prolidase activity. A melanin-induced augmentation of the inhibitory effects of gentamicin on collagen and DNA biosynthesis may explain the mechanism for the organ specificity of gentamicin-induced hearing loss in patients administered this drug.