CHARACTERIZATION OF ZETA-CRYSTALLIN INHIBITION BY JUGLONE

Guinea pig lens zeta-crystallin showed hyperbolic saturation curves wi th 9.10-phenanthrenequinone (PAQ), 5-hydroxy-1,4-naphthoquinone (juglo ne) and NADPH. Whereas camel lens zeta-crystallin showed hyperbolic sa turation curves only with PAQ and NADPH, bur slightly segmoidal with j uglone. For both enzymes PAQ was the preferred substrate. The catalyti c center activity (K-cat) values indicated that camel zeta-crystallin catalyzed the reduction of PAQ more efficiently than the guinea pig le ns zeta-crystallin, although the K-m values of the two enzymes for thi s quinone were very similar. The guinea pig lens zeta-crystallin catal yzed the reduction of Juglone far more efficiently than that of the ca mel lens zeta-crystallin. Juglone did not serve as an efficient substr ate 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 fo r guinea pig lens zeta-crystallin and camel lens zeta-crystallin, resp ectively. Thus, the camel lens zeta-crystallin was less active toward juglone as a substrate as well as less sensitive to its inhibitory act ion, when compared with guinea pig lens zeta-crystallin. The inhibitio n mechanism of guinea pig and camel lens zeta-crystallin by juglone is discussed. (C) 1996 Academic Press, Inc.