Studies on Amino Acid Activating Enzymes in Thyroid Glands1

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Abstract
Amino acid activation by heat labile enzymes in 105,000 Xg supernatant fluid of pig thyroid homogenates has been studied by measuring amino acyl (C14) hydroxamate formation and inorganic pyrophosphate (P32) exchange with ATP. The cell sap was used directly and after treatment with charcoal to minimize the endogenous ATP-PP; exchange reaction. L-Tyrosine, DL-tryptophan, L-monoiodotyrosine and DL-threonine stimulate the ATP-PPi exchange, whereas no effect, or slight inhibition, is observed with the common amino acids of proteins, D-tyrosine, L-diiodotyrosine, DL-thyronine, L-triiodothyronine and L-thyroxine. Hydroxamates of tyrosine, phenylalanine and methionine are formed with crude cell sap, although the rate of tyrosyl hydroxamate formation is only 1/5 as great as the tyrosine stimulated ATP-PPi exchange. Tyrosyl hydroxamate formation is inhibited 44 and 7 % by a 50-fold excess of L-monoiodotyrosine and L-diiodotyrosine, respectively, whereas thyronine and iodothyronines are without effect. A 10-fold excess of monoiodotyrosine inhibits tyrosyl hydroxamate formation by 27 %, but this amount of diiodotyrosine is not inhibitory. Tyrosine, but not iodotyrosines, is transferred to soluble RNA. (Endocrinology74: 273, 1964)