The Sulphydryl-Disulphide Hypothesis in Relation to Desiccation Injury of Cabbage Leaves

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Abstract
Sulphydryl (-SH) and disulphide (-SS-) levels were examined in extracts of cabbage leaves which had been desiccated to various degrees down to the death of the tissue at a water potential of -94 atm. The soluble protein fraction showed a progressive decrease, to 50% of the control, in "reactive" -SH, i. e. the -SH in the native protein capable of reacting with Ag+. About 60% of the decrease was attributed to con-figurational changes, since the remainder could be accounted for by a conversion of -SH to -SS- which appeared to involve the linkage of non-protein to the soluble protein. Associated with this linkage, a decrease occurred in the titratable sulphur of the non-protein fraction, without, however, any alteration in the proportion of -SH to -SS-. Structural protein displayed an increase in the "reactive" -SH apparently due to unfolding of the protein, at water potentials less than -40 atm (approx. 2: 5 water molecules/amino acid residue). The total -SH and total -SS-, however, remained unaltered until -94 atm (approx. 2: 3 water molecules/ amino acid residue), at which stage a considerable conversion of -SH to -SS- occurred, involving at least some protein-protein inter molecular bonds and accompanied by the death of the tissue. This latter process agrees with the predictions of Levitt''s sulphydryl-disulphide hypothesis.