Abstract
The conversion of thyroxine to tetraiodothyroacetic acid was studied employing rat kidney mitochondria as an enzyme source. The conversion was only slight without the addition of pyridoxal phosphate and α-ketoglutarate. The effect of these two cofactors of transaminases was much greater than that of any other compounds tested, such as DPN, succinate and malate. The enhancing effect of pyridoxal phosphate and amino acid acceptors (α-ketoglutarate, oxalacetate and pyruvate), only a slight influence of anaerobiosis, the lack of “dilution effect”, and a marked inhibitory effect of amino group donors (glutamate and aspartate) upon the conversion-all of these things indicated that the enzyme involved is of the nature of a transaminase rather than l-amino acid oxidase. The transaminase may be the main enzyme involved in the deaminative breaking down of thyroxine, even if other enzymes are present and active. The transaminase exists mainly within mitochondria and is inactivated by boiling for 20 minutes.
