Changes in sulfhydryl groups of honeybee glyceraldehyde phosphate dehydrogenase associated with generation of the intermediate plateau in its saturation kinetics.

Experiments with honeybee (Apis mellifera)and rabbit muscle GPDH were conducted to obtain information at the chemical level regarding anomolous saturation kinetics of the honeybee enzyme. Results demonstrate that the enzyme's sulfhydryl groups are implicated in the process. Measured by DTNB titration, native honeybee GPDH has one less active SH than the native rabbit muscle enzyme and displays changes in overall sulfhydryl reactivity after preincubation with G-3-P or G-3-P plus NAD + . The total DTNB reactive sulfhydryls of rabbit muscle GPDH are not changed by preincubation with NAD or G-3-P; honeybee GPDH, under certain conditions of preincubation with these ligands, shows a decrease of two total DTNB reactive SH groups. This difference has been confirmed by an independent experiment in which the two enzymes were carboxymethylated 14 with C-bromoacetic acid. The loss of SH groups in honeybee GPDH is not a result of its acylation by G-3-P. After generation of the stable form of the honeybee.enzyme with G-3-P plus NAD + it is possible to regenerate the anomolous kinetic curve by ti:eatment of the enzyme with dithiothreitol. It is proposed that in honeybee GPDH, an intrachain disulfide bond forms in conjunction with the conversion of the enzyme from the "metastable" to stable state.