Role of hydration state and thiol-disulfide status in the control of thermal stability and protein synthesis in wheat embryo

Abstract

Reduced (GSH), oxidized (GSSG), and protein-bound (PSSG) glutathione were determined in dry and hydrated wheat embryos. Dry embryos contained about 0.6 mumoles per gram dry weight each of GSSG and PSSG, and these levels declined 5- to 10-fold within minutes after the onset of imbibition. GSH declined from about 8 to 2 mumoles per gram over a period of 90 minutes. Similar changes occurred when embryos were hydrated by storage at 100% relative humidity. The decline in glutathione levels was not reversed upon redrying hydrated embryos. About 40% of the cysteine residues of embryo protein was found to be in the disulfide form in both dry and imbibed embryos. The ability of wheat embryos to withstand heat shock was shown to correlate with water content but not GSSG content. Incorporation of [(35)S]methionine into protein was studied using a system based upon wheat embryo extract (S23). Incorporation rate was found to be sensitive to the nature of thiol added to the system and to be decreased by GSSG. S23 exhibited a substantial capacity to reduce GSSG and preparation of S23 having a GSSG content comparable to dry embryos required addition of large amounts of GSSG to the extraction buffer S23 prepared in this fashion exhibited a marked decrease in ability to support protein synthesis. These results suggest that the early decrease in GSSG during germination is necessary for optimal protein synthesis in wheat embryo.