INHIBITION OF H-CEREVISIAE BY INSOLUBLE METAL PHOSPHATES AND PROTECTION BY CALCIUM AND MAGNESIUM - INHIBITORY EFFECTS A RESULT OF SOLUBLE METAL-CATIONS( EFFLUX FROM SACCHAROMYCES)

Insoluble metal phosphates were capable of inhibitory effects on plasm a membrane H+-ATPase-mediated glucose-dependent H+ efflux from Sacchar omyces cerevisiae. The relative toxicities of the different compounds used was Zn > Co which was the same order as that obtained for soluble chlorides of these metals, although the toxicity of the latter was ma nifest at lowe; concentrations. Inhibitory effects of the metal phosph ates were dependent on the amount in suspension and increased with inc reasing preincubation time. The inhibitory effect of toxic metal compo unds on glucose-dependent H+ efflux could be reduced or prevented by t he addition of chlorides of calcium and magnesium, calcium being more effective than magnesium. The relative protective effects of Ca and Mg were similar for both soluble and insoluble compounds, and Ca could n eutralize the influence of both cobalt and zinc phosphates. Although t oxic effects could result from the solubilization of the insoluble met al phosphate compounds as a result of H+ efflux, releasing potentially toxic metal cations, there was no difference in the amount of metal c ations released whether glucose was present or absent from the treatme nts. In addition, it was found that significant concentrations of solu ble Co2+ and Zn2+ were released into metal phosphate suspensions in th e absence or presence of cells with equilibrium concentrations in cell free suspensions (attained after 10-20 min) being about 60 mu M for Z n-3(PO4)(2) and about 65 mu M for Co-3(PO4)(2). In the presence of cel ls, a lower equilibrium concentration was attained in the presence of Zn, probably due to uptake by glucose-dependent and -independent proce sses. Such uptake was not observed for released Co2+ and could account for the higher toxicity of zinc phosphate compared to cobalt phosphat e. Although some direct interactions with the insoluble phosphates mus t have been involved in overall inhibitory effects, it is concluded th at a significant proportion of toxic effects resulted from free metal cations in solution, with protective effects of Ca and Mg resulting fr om competitive and stabilizing interactions at the cell surface.