The age and species dependent characteristics of cutaneous esterase activity were examined in cultured keratinocytes of neonatal and adult humans and of rats at the age of 1, 3, 10, and 50 d. The existence of esterases was characterized using fluorescein-5-isothiocyanate diacetate under a confocal laser scanning microscope. In vitro hydrolysis of ethyl nicotinate (EN), an esterified prodrug of nicotinic acid, was investigated in homogenate of cultured keratinocytes, and the Michaelis-Menten

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... meters (V max and K m ) of EN were evaluated. Together with development and growth of rats and humans, V max and V max /K m increased drastically, suggesting that esterases in keratinocytes develop markedly during the growth process. The affinity parameter, K m , was almost the same among the ages in each species. These findings in cultured keratinocytes corresponded with our previous report using dissected skin specimens. Species differences in V max , V max /K m and K m were also observed, and these parameters of EN hydrolysis in rats were significantly higher than that in humans. In conclusion, cultured keratinocytes can be an advantageous method with which to estimate cutaneous activation of ester prodrugs in humans and during the growth process. Key words keratinocyte; age dependency; metabolism; ethyl nicotinate; development; species difference a) Each value represents the meanϮS.D. of three experiments. b) Normal human epidermal keratinocytes were obtained from a male newborn neonate and from a 51-yearold male (adult). * pϽ0.05 compared with rat at the age of 1 and 3 d, or human neonate.