Esterase hydrolysis of an homologous series of PABA esters in hairless mouse and human skin homogenates.

Abstract

This thesis addresses the extent to which esters formed from homologous alcohols are metabolized in skin and examines if metabolism in hairless mouse skin parallels that in human skin and thus offers a useful membrane model for studying metabolic processes. Such a model membrane is desirable for investigating aspects of transdermal and topical delivery. Although much work in the percutaneous absorption area has been done using hairless mouse skin, few researchers have made comparisons of this skin to that of man in these regards. To compare the membranes, the metabolism of a homologous series of esters of p-aminobenzoic acid was studied in skin homogenates. Hairless mouse skin homogenates have the advantage of being a relatively quick method of assessing cutaneous metabolism and , being a readily available source of skin, metabolism within this tissue can be extensively characterized. Murine homogenate data can be compared with data related to transport through the intact membrane, another important aspect of transdermal delivery, to get a relatively complete picture of the metabolic influences. Each of the homologous p-aminobenzoates, the methyl through the pentyl, was incubated with hairless mouse skin homogenates to determine their apparent K$\\sb{\\rm m}$, V$\\sb{\\rm max}$ and V$\\sb{\\rm max}$/K$\\sb{\\rm m}$ ratios. Generally, the longer chain esters were more readily hydrolyzed by hairless mouse skin. Human skin from a variety of sources was studied for its enzymatic activity against the same compounds. Activity was only seen when fresh surgical skin was used. Moreover, short chain esters were the most readily cleaved. It was concluded from this work that the esterase activity of human skin is very different from that of hairless mouse skin both in chain-length specificity and rate of hydrolysis. The smaller extent of hydrolysis seen with human skin indicates that either the enzyme in human skin is much less active or is present in much smaller quantities than the enzyme in hairless mouse skin. The results presented here point to the need to thoroughly check model systems against the human system before attempting to extrapolate results to human therapeutics.