[Physico-chemical properties of ribose-5-phosphate isomerase from spinach leaves].

An electrophoretically homogenous D-ribose-5-phosphate-ketol-isomerase (Ec 5.3.1.6) from spinach leaves was obtained. The effects of substrate, temperature, pH and some agents on the enzyme activity were studied and the kinetic parameters were calculated. The pK values for the monogenic groups of the enzyme were found equal to 7.40, 7.95 and 8.80. It was assumed that the active site of the enzyme contains several alpha-amino groups and a SH-group. pCMB titrates about 2 moles of SH-groups per mole of enzyme, while 5,5'-dithiobis(2-nitrobenzoate)--less than 1 mole of SH-groups per mole of enzyme.