Metabolic Difference between 3, 4-Dihydroxyphenylpyruvic Acid (DHPP) and 3-Methoxy-4-hydroxyphenylpyruvic Acid (MHPP)

Abstract

3, 4-Dihydroxyphenylpyruvic acid (DHPP) was mainly metabolized to L-3, 4-dihydroxyphenylalanine (L-DOPA) through transamination, while 3-methoxy-4-hydroxy-phenylpyruvic acid (MHPP) was mainly metabolized to 3-methoxy-4-hydroxyphenyllactic acid (MHPL) through reduction. In order to explain the metabolic difference between these two aromatic α-keto acids related with DOPA, the metabolism of ¹⁴C-labeled DHPP and MHPP was compared in vitro. There was no significant difference between the two α-keto acids in the initial rate of transamination to the corresponding amino acids using rat liver mitochondrial fraction. The K_m values to mitochondrial transaminase were calculated to be 8 mM and 6 mM for DHPP and MHPP, respectively. NADH-dependent activity for reducing DHPP and MHPP to the corresponding aromatic α-hydroxy acids was the highest in the heart followed by the kidney, muscle and liver and was not inhibited with oxamate (5 mM), the inhibitor of lactate dehydrogenase, indicating that aromatic α-keto acid reductase (AKAR) predominantly participates in this reaction. The initial rate of DHPP reduction with purified rat liver AKAR preparation was, however, only about 1/10 of that of MHPP reduction. The K_m value of DHPP (3.6 mM) was about two-fold larger than that of MHPP (1.5 mM), demonstrating a lower affinity of DHPP to AKAR than that of MHPP.