Freezing injury in spinach leaf tissue: Effects on water‐soluble proteins, protein‐sulfhydryl and water‐soluble non‐protein‐sulfhydryl groups

Freezing of spinach leaf discs ( Spinacia aleracea L. cv. Estivato) resulted in an irreversible and parallel loss of protein‐sulfhydryl (SH) and water‐soluble protein. This decrease was inversely related to the increase in freezing injury as determined by the loss of electrolytes from the tissue after thawing. Loss of proteins and protein‐SH occurred during freezing of the tissue and was not enhanced by thawing. The parallel decreases in content of soluble proteins and SH groups make it impossible to determine whether oxidation of protein‐SH groups is the primary step in decline of protein content. During freezing the content of non‐protein‐SH compounds, mainly glutathione (GSH), was decreased to a lesser extent than that of protein‐SH. Contrary to protein‐SH, the levels of non‐protein‐SH declined substantially after thawing. The data indicate that GSH is not directly involved in protection of soluble proteins against freezing‐induced denaturation.