Age-related effects on subunit interactions in rat muscle glyceraldehyde-3-phosphate dehydrogenase

Mapping tissue-specific genes correlated with age-dependent changes in protein stability and function

Biophysical measurements indicative of protein stability and function were performed on crude extracts from liver, muscle, and lens of a genetically heterogeneous mouse population. Genetic information was used to search for quantitative trait loci (QTL) that influenced the biophysical traits, with emphasis on phenotypes that previously have been shown to be altered in aged animals. Spectroscopic and enzymatic assays of crude liver and muscle tissue extracts from approximately 600 18-month-old mice, the progeny of (BALB/cJxC57BL/6J)F1 females and (C3H/HeJxDBA/2J)F1 males, were used to measure the susceptibility of a ubiquitous glycolytic enzyme, glyceraldehyde-3-phosphate dehydrogenase (GAPDH), to thermal denaturation. The rate constant for thermal inactivation of GAPDH correlated with markers on chromosome 5 (D5Mit79 and D5Mit251) for muscle lysates and chromosome 15 (D15Mit63 and D15Mit100) for liver tissue. The degree of variability of inactivation rate constants, a measure of the heterogeneity of muscle GAPDH in tissue extracts, was also associated with markers on chromosome 5 (D5Mit79 and D5Mit205). In addition, spectroscopic characteristics of extracted eye lens proteins were evaluated for their susceptibility to photooxidative stress. Absorbance and fluorescence emission characteristics of the lens proteins were mapped to QTL on chromosomes 5 and 15 (D5Mit25 and D15Mit171) while the degree of heterogeneity in photochemical oxidation kinetics was associated with a marker on the chromosome 8 (D8Mit42). Recent work has shown that GAPDH possesses a number of non-glycolytic functions including DNA/RNA binding and regulation of protein expression. Tissue specific differences in GAPDH stability may have significant consequences to these alternate functions during aging.

Photo-oxidative damage to lenticular GAPDH and its relationship to aldehyde metabolism

These studies show that GAPDH from the calf lens and human lens epithelium undergoes size and charge modifications in the presence of active oxygen species generated from methylene blue and light. The histidine content of the enzyme was reduced by one third. No evidence of increased proteolysis was observed suggesting a nonenzymatic cleavage of enzyme by the photo-oxidants.

Mechanism of aging of rat muscle glyceraldehyde-3-phosphate dehydrogenase studied by selective enzyme-oxidation

Controlled oxidation of rat muscle glyceraldehyde-3-phosphate dehydrogenase (GPDH) was carried out in an attempt to stimulate age-related effects observed in enzyme samples purified from old animals. A comparative study of the "simulated aged" and of native young and old GPDH forms was done using fluorescence techniques. The present work is based on our previous findings that the locus of the age-related modifications in GPDH is in the nicotinamide-binding site, where the catalytically active Cys-149 residue is located, and that an increase in oxidation potential occurs in old animal tissues which may enable various oxidizing agents to play a significant role in the inactivation of certain enzymes. Thus it has been suggested that the loss of specific activity observed in old GPDH may be due to subtle and irreversible conformational changes caused by reaction of Cys-149 with these agents. The circularly polarized luminescence (CPL) spectrum emitted by the fluorescent sulfhydryl reagent I-AEDANS covalently bound to GPDH through Cys-149 at the nicotinamide binding site, revealed a significant difference in conformation between these sites in young and old GPDH forms. Large differences were also observed between corresponding spectra when the binding sites were saturated with NAD+, reflecting the development of marked conformational changes in both young and old GPDH species upon coenzyme binding. The oxidizing reagents employed in the current study (hydrogen peroxide, superoxide radical and atmospheric dioxygen) are all expected to be more commonly encountered in the less reducing environment of old animal tissues. All of them, though to a different extent, caused a significant inactivation of the enzyme dependent on the initial oxidant concentration. Although the original enzymatic activity could be partially restored by incubation with a reducing agent, the prior oxidation was found to induce some irreversible structural changes as expressed in a decrease in the number of fast reacting SH groups. The extent of irreversible inactivation was a function of both oxidant concentration and the duration of exposure to the oxidant. The affinity of the oxidized GPDH species (termed "aged") toward coenzyme, as monitored by fluorometric titrations, was markedly lower than that observed for both the native young and old GPDHs. In addition, the CPL spectra of the "aged" enzymes were different from those obtained for both native forms.(ABSTRACT TRUNCATED AT 400 WORDS)