Glutamic dehydrogenase from rat heart mitochondria. I. Purification and physical properties including molecular weight determination

Pyruvate dehydrogenase and the regulation of glucose oxidation in hypertrophied rat hearts

OBJECTIVE

Coupling of glucose oxidation to glycolysis is lower in hypertrophied than in non-hypertrophied hearts, contributing to the compromised mechanical performance of hypertrophied hearts. Here, we describe studies to test the hypothesis that low coupling of glucose oxidation to glycolysis in hypertrophied hearts is due to reduced activity and/or expression of the pyruvate dehydrogenase complex (PDC).

METHODS

We examined the effects of dichloroacetate (DCA), an inhibitor of PDC kinase, and of alterations in exogenous palmitate supply on coupling of glucose oxidation to glycolysis in isolated working hypertrophied and control hearts from aortic-constricted and sham-operated male Sprague-Dawley rats. It was anticipated that the addition of DCA or the absence of palmitate would promote PDC activation and consequently normalize coupling between glycolysis and glucose oxidation in hypertrophied hearts if our hypothesis was correct.

RESULTS

Addition of DCA or removal of palmitate improved coupling of glucose oxidation to glycolysis in control and hypertrophied hearts. However, coupling remained substantially lower in hypertrophied hearts. PDC activity in extracts of hypertrophied hearts was similar to or higher than in extracts of control hearts under all perfusion conditions. No differences were observed between hypertrophied and control hearts with respect to expression of PDC, PDC kinase, or PDC phosphatase.

CONCLUSIONS

Low coupling of glucose oxidation to glycolysis in hypertrophied hearts is not due to a reduction in PDC activity or subunit expression indicating that other mechanism(s) are responsible.

Association-dissociation studies of bovine and rat liver glutamic dehydrogenase by high-performance liquid chromatography gel filtration

The concentration-dependent association-dissociation tendency of purified bovine liver and rat liver glutamic dehydrogenase (GDH) has been demonstrated by high-performance liquid chromatographic gel filtration. In the concentration range of 100 to 1.0 micrograms bovine GDH/ml molecular species ranged from dimer and unimer to subunimeric forms. The dissociation process of the unimeric hexapeptide, consisting of six polypeptide chains, to the subunimeric tripeptide, consisting of three polypeptide chains, was irreversible without added ionic support, but reversible with added ionic support. In dilute Tris-HCl bovine liver GDH was dispersed to subunimeric sizes. Increasing the ionic strength in 20 mM phosphate as the mobile phase increased dissociation to a subunimeric tripeptide while sustaining as much as 80% of its activity. Activity of a eluting subunimer was verified by the inclusion of reaction substrates (NAD and glutamute) in the mobile phase and quantification of reaction products (NADH) in chromatograms. Gel filtration of GDH in the presence of GTP with NADH rendered a subunimeric tripeptide, largely independent of ionic strength or GDH concentration. Rat liver GDH, differing from bovine liver GDH, was dissociated by gel filtration to an active tripeptide independent of ionic or buffer conditions.