Age-dependent changes of protein structure. I. tissue-specific, electrophoretic and catalytical properties of muscle aldolase of old rabbits

Age-related effects in enzyme catalysis

Rat muscle glyceraldehyde-3-phosphate dehydrogenase is one of several enzymes which have been found to undergo age-related modifications. While the amount of this enzyme in muscle tissue does not change with age, both its specific activity and affinity towards its co-enzyme are significantly reduced in the old tissue. Age-related structural changes were found to exist in the nicotinamide binding site of the enzyme and the reactions leading to the activity loss in 'old' glyceraldehyde-3-phosphate dehydrogenase were shown to involve a reversible modification of the essential cysteine-149 residue at the active site of the enzyme. The aging effects were simulated by a controlled oxidation of cys-149 in samples of 'young' glyceraldehyde-3-phosphate dehydrogenase and subsequent reduction of this residue by 2-mercaptoethanol. The enzyme modified in this way closely resembles native 'old' glyceraldehyde-3-phosphate dehydrogenase, indicating that the structural modifications in the latter enzyme are indeed introduced by a post-translational process. The mechanism for aging of glyceraldehyde-3-phosphate dehydrogenase which is proposed, based on these observations, thus assumes an oxidation of cys-149 as its first step followed by irreversible conformational changes in the enzyme molecule. The aging of glyceraldehyde-3-phosphate dehydrogenase may thus be triggered by the reduced ability of old muscle tissue to protect its constituents against oxidation.

Age-dependent changes of protein structure. The properties of young and old rabbit aldolase are restored after reversible denaturation

The significantly increased helical content is observed in muscle aldolase molecule of old rabbits. The unfolding and refolding of protein conformation followed by circular dichroism, fluorescence and enzyme activity showed the recovery of initial conformation after the denaturation. The protein folds into the form that existed prior to denaturation--"young" into "young" and "old" into "old"--the conformational differences between them being restored. This suggests that the primary structure modifications prior to the folding of the native protein conformation are the origin of the age-dependent differences of aldolase structure and function.

Age-related effects in coenzyme binding patterns of rat muscle glyceraldehyde-3-phosphate dehydrogenase

The binding of NAD+ and of its fluorescent analogue, nicotinamide 1,N6-ethenoadenine dinucleotide, to glyceraldehyde-3-phosphate dehydrogenase purified from the muscles of young and old rats was studied in detail. Binding of the natural coenzyme was followed both by spectrophotometric titration of the extrinsic absorption band of the enzyme-NAD+ complex and from the degree of quenching of fluorescence of the protein. Binding of the coenzyme analogue was monitored by using the large enhancement in its fluorescence upon forming the complex with the enzyme. Both dinucleotides showed strong negative cooperativity in binding to the enzyme, similar to that displayed in their association with the rabbit muscle enzyme. The enzyme purified from old rats displayed a markedly reduced affinity toward the two dinucleotides, compared with the enzyme isolated from young animals. The various dissociation constants of both dinucleotides from the enzyme from young rats were remarkably similar to the corresponding constants in the rabbit muscle enzyme. The degree of negative cooperativity (i.e., the ratio between the dissociation constants from high- and low-affinity binding sites) in the "young" and "old" enzyme forms was not very different. It was concluded from these results that while modifications in the subunits take place upon aging, the intersubunit interaction is not significantly affected. Increasing concentrations of ATP were found to cause a gradual decrease in the negative cooperativity of NAD+ binding, which completely disappeared in the presence of 5 mM ATP. The observation that all four binding sites of the old enzyme display the same affinity toward NAD+ when the negative cooperativity is removed excludes the possibility that this enzyme form is a mixture of native and inactive species. The different dissociation constants of NAD+ from young and old enzyme forms in the presence of 5 mM ATP also demonstrate the occurrence of age-related modifications in the structure of the individual subunits.