Mechanism for proliferation inhibition by various selenium compounds and selenium-enriched broccoli extract in rat glial cells.

The objective of this study was to investigate the differential effects of various selenium (Se) compounds and Se-enriched broccoli extracts on cell proliferation and the possible mechanism responsible for the Se-induced growth inhibition. C6 rat glial cells were incubated with graded concentrations up to 1000 nM of selenite, selenate, selenomethionine (SeM), Se-methyl-selenocysteine (SeMCys), high-Se broccoli (H-SeB) extract or low-Se broccoli (L-SeB) extract for 24 and 48 h. MTT results indicated that all Se sources and levels examined inhibited C6 cell proliferation at 48 h. The results from cell cycle progression and apoptosis analysis indicated that SeM, SeMCys, H-SeB or L-SeB treatments at the concentration of 1000 nM reduced the cell population in G(0)/G(1) phase, but induced G(2)/M phase arrest and increased apoptosis and secondary necrosis in C6 cells at 24 h. The populations of apoptotic cells and secondary necrotic cells were increased by all Se sources examined. The COMET assay indicated that there was no significant DNA single-strand break found for all Se treatments in C6 cells for 48 h. In addition, the Se-induced proliferation inhibition may involve a hydrogen peroxide (H(2)O(2))-dependent mechanism with elevated cellular glutathione peroxidase (cGPX) activity. Both H-SeB and L-SeB inhibited C6 cell proliferation but H-SeB was less inhibitory than L-SeB. The proliferation inhibition by H-SeB in C6 cells is apparently related to the increased H(2)O(2) with the elevated cGPX activity, but the inhibition by L-SeB was H(2)O(2)-independent without change in cGPX activity.

pH Dependence of peptidylglycine monooxygenase. Mechanistic implications of Cu-methionine binding dynamics.

The pH dependence of the PHM-catalyzed monooxygenation of dansyl-YVG was studied in two different buffer systems in the pH range of 4-10. The pH-activity profile measured in a sulfonic acid buffer exhibited a maximum at pH 5.8 and became inactive at pH >9. The data could be fit to a model that assumed a protonated unreactive species A, a major reactive species B, and a less reactive species C. B formed in a deprotonation step with pK(a) of 4.6, while C formed and decayed with pK(a)s of 6.8 and 8.2, respectively. The pH dependence was found to be dominated by k(cat), with K(m)(dansyl-YVG) remaining pH-independent over the pH range of 5-8. Acetate-containing buffers shifted the pH maximum to 7.0, and the activity-pH profile could be simulated by formation and decay of a single active species with pK(a)s of 5.8 and 8.3, respectively. The pH-dependent changes in activity could be correlated with a change in the Debye-Waller factor for the Cu-S(met) (M314) component of the X-ray absorption spectrum which underwent a transition from a tightly bound inactive "met-on" form to a conformationally mobile active "met-off" form with a pK(a) which tracked the formation of the active species in both sulfonic acid and acetate-containing buffer systems. The data suggested that the conformational mobility of the bound substrate relative to the copper-superoxo active species is critical to catalysis and further suggested the presence of an accessible vibrational mode coupling Cu-S motion to the H tunneling probability along the Cu-O...H...C coordinate.