LIGHT-DEPENDENT INCORPORATION OF SELENITE AND SULFITE INTO SELENOCYSTEINE AND CYSTEINE BY ISOLATED PEA-CHLOROPLASTS

Illuminated intact pea chloroplasts in the presence of O-acetylserine (OAS) catalysed incorporation of SeO32- and SO32- into selenocysteine and cysteine at rates of ca 0.36 and 6 μmol/mg Chl per hr respectively. Sonicated chloroplasts catalysed SeO32- and SO32- incorporation at ca 3.9 and 32% respectively of the rates of intact chloroplasts. Addition of GSH and NADPH increased the rates to ca 91 and 98% of the intact rates, but SeO32- incorporation under these conditions was essentially light-independent. In the absence of OAS, intact chloroplasts catalysed reduction of SO32- to S2- at rates of ca 5.8 μmol/mg Chl per hr. In the presence of OAS, S2- did not accumulate. Glutathione (GSH) reductase was purified from peas and was inhibited by ZnCl2. This enzyme, in the presence of purified clover cysteine synthase, OAS, GSH and NADPH, catalysed incorporation of SeO32- into selenocysteine (but not SO32- into cysteine). The reaction was inhibited by ZnCl2. Incorporation of SeO32- into selenocysteine by illuminated intact chloroplasts and sonicated chloroplasts (with NADPH and GSH) was also inhibited by ZnCl2 but not by KCN. Conversely, incorporation of SO32- into cysteine was inhibited by KCN but not by ZnCl2. It was concluded that SeO32- and SO32- are reduced in chloroplasts by independent light-requiring mechanisms. It is proposed that SeO32- is reduced by light-coupled GSH reductase and that the Se2- produced is incorporated into selenocysteine by cysteine synthase. © 1979.