Characterization of zeta-crystallin inhibition by juglone

Guinea pig lens zeta-crystallin showed hyperbolic saturation curves with 9.10-phenanthrenequinone (PAQ), 5-hydroxy-1,4-naphthoquinone (juglone) and NADPH. Whereas camel lens zeta-crystallin showed hyperbolic saturation curves only with PAQ and NADPH, bur slightly segmoidal with juglone. For both enzymes PAQ was the preferred substrate. The catalytic center activity (K-cat) values indicated that camel zeta-crystallin catalyzed the reduction of PAQ more efficiently than the guinea pig lens zeta-crystallin, although the K-m values of the two enzymes for this quinone were very similar. The guinea pig lens zeta-crystallin catalyzed the reduction of Juglone far more efficiently than that of the camel lens zeta-crystallin. Juglone did not serve as an efficient substrate for both zeta-crystallins compared to PAQ and appeared to act as a potent competitive inhibitor, with K-l values of 75 nM and 20 mu M for guinea pig lens zeta-crystallin and camel lens zeta-crystallin, respectively. Thus, the camel lens zeta-crystallin was less active toward juglone as a substrate as well as less sensitive to its inhibitory action, when compared with guinea pig lens zeta-crystallin. The inhibition mechanism of guinea pig and camel lens zeta-crystallin by juglone is discussed. (C) 1996 Academic Press, Inc.