Studies on tyrosine-alpha-ketoglutarate transaminase from bovine thyroid and liver tissue

1. 1.This paper describes the purification and characteristics of tyrosine-α-ketoglutarate transaminase ( l -tyrosine:2-oxoglutarate aminotransferase, EC 2.6.1.5) from bovine thyroid tissue. The most active preparations appeared to be more than 80% homogeneous on the basis of ultracentrifugal and immunoelectrophoretic analysis. 2. 2.Bovine thyroid tyrosine-α-ketoglutarate transaminase was compared with the same enzyme isolated from bovine liver and found to be similar with respect to heat tolerance, pH optima, substrate affinity, and response to most inhibitors. Immunologically the proteins appeared to be identical. 3. 3.The only difference between the two enzymes was the effect of diiodotyrosine. Diiodotyrosine did not inhibit thyroid transaminase and would partially protect it against inhibition by the sulfhydryl inhibitor, p -chloromercuriphenylsulfonic acid. On the other hand, diiodotyrosine inhibited liver transaminase 50% at 2.5 mM concentration and did not protect it from the inhibitory effect of p -chloromercuriphenylsulfonic acid. 4. 4.Both thyroid and liver transaminase utilized tyrosine and monoiodotyrosine as substrates but used diiodotyrosine poorly.