Human antiquitin: structural and functional studies.
ABSTRACT Antiquitin (ALDH7) is a member of the aldehyde dehydrogenase superfamily which oxidizes various aldehydes to form the corresponding carboxylic acids. Human antiquitin (ALDH7A1) is believed to play a role in detoxification, osmoregulation and more specifically, in lysine metabolism in which alpha-aminoadipic semialdehyde is identified as the specific, physiological substrate of the enzyme. In the present study, the structural basis for the substrate specificity was studied by site-directed mutagenesis. Kinetic analysis on wild-type human antiquitin and its mutants E121A and R301A demonstrated the importance of Glu121 and Arg301 in the binding as well as the turnover of alpha-aminoadipic semialdehyde. On the functional aspect, in addition to the already diversified physiological functions of antiquitin, the recent demonstration of its presence in the nucleus suggests that it may also play a role in cell growth and cell cycle progression. In this investigation, the expression level of antiquitin was monitored in synchronized WRL68 and HEK293 cell culture systems. It was found that the protein was up-regulated during G(1)-S phase transition. Immunofluorescence staining of the synchronized cells demonstrated an increased expression and accumulation of antiquitin in the nucleus during the G(1)-S phase transition. Knockdown of antiquitin using shRNA transfection also resulted in changes in the levels of several key cell cycle-regulating proteins.