HOMOGENTISIC ACID OXIDASE

Steady-state kinetics and inhibition of anaerobically purified human homogentisate 1,2-dioxygenase

HGO (homogentisate 1,2-dioxygenase; EC 1.13.11.5) catalyses the O2-dependent cleavage of HGA (homogentisate) to maleylacetoacetate in the catabolism of tyrosine. Anaerobic purification of heterologously expressed Fe(II)-containing human HGO yielded an enzyme preparation with a specific activity of 28.3+/- 0.6 micromol x min(-1) x mg(-1) (20 mM Mes, 80 mM NaCl, pH 6.2, 25 degrees C), which is almost twice that of the most active preparation described to date. Moreover, the addition of reducing agents or other additives did not increase the specific activity, in contrast with previous reports. The apparent specificity of HGO for HGA was highest at pH 6.2 and the steady-state cleavage of HGA fit a compulsory-order ternary-complex mechanism (K(m) value of 28.6+/-6.2 microM for HGA, K(m) value of 1240+/-160 microM for O2). Free HGO was subject to inactivation in the presence of O2 and during the steady-state cleavage of HGA. Both cases involved the oxidation of the active site Fe(II). 3-Cl HGA, a potential inhibitor of HGO, and its isosteric analogue, 3-Me HGO, were synthesized. At saturating substrate concentrations, HGO cleaved 3-Me and 3-Cl HGA 10 and 100 times slower than HGA respectively. The apparent specificity of HGO for HGA was approx. two orders of magnitude higher than for either 3-Me or 3-Cl HGA. Interestingly, 3-Cl HGA inactivated HGO only twice as rapidly as HGA. This contrasts with what has been observed in mechanistically related dioxygenases, which are rapidly inactivated by chlorinated substrate analogues, such as 3-hydroxyanthranilate dioxygenase by 4-Cl 3-hydroxyanthranilate.

Profiles in altered metabolism. II.--Accumulation of homogentisic acid in serum and urine following acetylsalicylic acid ingestion

Homogentisic acid elevations have been found in urine and serum of patients on long term heavy salicylate therapy and those acutely intoxicated with acetylsalicylic acid. Loading of two normal volunteers with single oral acetylsalicylate doses produced transient elevations in urinary homogentisic acid excretion. These findings suggest that heavy salicylate use results in the partial inhibition of homogentisic acid oxidase in vivo.

Experimental alkaptonuria in the rat induced by tryptophan deficiency

An experimental alkaptonuria induced by tryptophan deficiency has been observed in the rat. The condition of alkaptonuria followed development of signs of tryptophan deficiency as judged by weight loss of the young, growing rat and its general appearance. Homogentisic acid was identified by a comparison of its behavior on thin-layer chromatography with authentic homogentisate and by means of the enzyme homogentisic acid oxidase. The condition was relieved by the return of tryptophan to the diet.