Effects of Thyroid Hormones on the Multiple Molecular Forms of Mitochondrial Malate-Dehydrogenase

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Abstract
The effect of the thyroid hormones on the configurational state of mitochondrial malate-dehydrogenase has been investigated. A new, heavy form of malate-dehydrogenase (9 S) has been purified from pig-heart mitochondria. The physicochemical and chemical properties of this enzyme, compared with those of the well-known mitochondrial enzyme (5 S), show that 9S protein differs from the lighter form by: (a) content of sulfydryl groups, (b) the presence of tryptophan, (c) immunochemically. The configuration of this new form of malate-dehydrogenase appears to be that of a tetramer composed of two non-identical subunits. Low concentrations of thyroxine and iodine cyanide are able to dissociate the 9 S enzyme into smaller subunits whose sedimentation coefficients approximate 5 S. The disaggregation of the 9 S protein is coincident with an enhancement of the enzyme activity in the order of 50% over the initial value. The degree of activation is strongly influenced by the pH of incubation, the greatest effect being obtained at pH ranging between 7.0 and 7.4. One of the subunits, the ‘catalytic’, shows physico-chemical and immunochemical properties similar to those of the well-known mitochondrial enzyme (5 S). The other, ‘regulatory’ subunit, is devoided of enzyme activity and is immunochemically different from the ‘catalytic’ one. A scheme suggesting the allosteric effect of thyroxine in the regulation of malate-dehydrogenase activity is presented.

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