Conversion of ribulose-1,5-bisphosphate carboxylase to an acidic and catalytically inactive form by extracts of osmotically stressed Lemna minor fronds.

The fronds of Lemna minor L. respond to a number of stresses, and in particular to an osmotic stress, by producing an enzyme system which catalyzes the oxidation of ribulose-1,5-bisphosphate carboxylase (RuBPCase; EC 4.1.1.39) to an acidic and catalytically inactive form. During the first 24 h of osmotic stress the induced oxidase system does not seem to exert a significant in-vivo effect on RuBPCase, presumably because of compartmentation. Subsequently, the oxidase system gains access to the enzyme and converts it to the acid and catalytically inactive form and eventually the oxidase system declines in activity.A number of partially acidified forms of RuBPCase are formed during oxidation, and this process appears to be correlated with the disappearance of varying numbers of SH residues. The number of-SH residues in RuBPCase from Lemna has been estimated at 89. However, RuBPCase isolated from 24-h osmotically stressed fronds showed a reduction in the number of-SH residues per molecule from 89 to 54. It seems likely that the oxidation of-SH groups is causally related to the acidification of RuBPCase which occurs during osmotic stress.