Purification and comparative study of glyceraldehyde-3-phosphate dehydrogenase from the muscles of young and old rats

D-Glyceraldehyde-3-phosphate dehydrogenase (GPDH) was purified from the muscles of young and old rats. A marked difference was found between the (total) activities of these two enzyme preparations which originates in their different specific activities, while the concentrations of the enzyme in young and old tissues appear to be similar. Both "young" and "old" enzyme forms show four rapidly reacting sulfhydryl groups while six additional SH groups are revealed upon longer incubation with the sulfhydryl reagent. The UV absorption spectra and sedimentation coefficients of young and old GPDH molecules are also identical, and while the two enzyme forms partially dissociate into dimers in the presence of sodium chloride, the old enzyme appears to be more dissociable. The amino acid compositions of the GPDH molecules purified from young and old rats are remarkably alike and show a great similarity to the compositions of GPDH molecules from other mammals. Small differences in composition may, however, have escaped detection due to accuracy limitations of the determination and could be responsible for the differences in enzymatic activity. Alternatively, the activity differences may originate in postsynthetic conformational changes induced in the old enzyme during its longer "dwell time" in the tissue.