Acetyl-L-tyrosine hydrazide

We have investigated oxidation of amino acid phenylhydrazides by mushroom tyrosinase in the presence of 4-tert-butylcatechol and N-acetyl-L-tyrosine. Spectrophotometric measurements showed gradual disappearance of 4-tert-butyl-o-benzoquinone, generated by oxidation of 4-tert-butylcatechol with sodium periodate, after addition of amino acid phenylhydrazides. However, the presence of the phenylhydrazides did not influence the concentration of 4-tert-butyl-o-benzoquinone formed during enzymatic oxidation. Oxygen consumption measurements demonstrated that in a mixture both compounds were oxidized but the reaction rate was proportional to the concentration of the catechol. In the oxidation of N-acetyl-L-tyrosine addition of phenylhydrazides shortened the lag period, indicating that they acted as reducing agents, converting N-acetyl-L-dopaquinone to N-acetyl-L-dopa. In HPLC analysis of the oxidation 4-tert-butylcatechol and the phenylhydrazide of Boc-tryptophan only the N-protected amino acid and 4-tert-butyl-o-benzoquinone were detected as final products. In the presence of the natural substrates the oxidation of amino acid phenylhydrazides required much smaller amounts of the enzyme and was up to 40 times faster than the reaction carried out without these compounds. These results demonstrate that tyrosinase can oxidize phenylhydrazides indirectly through o-quinones. This reaction explains the inhibitory effect of agaritine, a natural amino acid hydrazide, on melanin formation and the inhibitory effects of other hydrazine derivatives on tyrosinase described in the literature.

The interaction of tyrosinase with the anticancer drug procarbazine has been investigated. In the presence of the enzyme alone no oxidation of this dialkylhydrazine above the background level was observed. However, when phenolic substrates (4-tert-butylcatechol or N-acetyl-l-tyrosine) were included in the reaction mixture, procarbazine was rapidly degraded. Oxygen consumption measurements showed that in a mixture both the phenolic substrate and the drug were oxidized. The major product of procarbazine degradation was isolated and identified as azoprocarbazine, the first active metabolite of this drug detected in previous in vivo and in vitro studies. This indirect oxidation of the hydrazine group in this anticancer agent indicates possible application of a hydrazine linker in construction of tyrosinase-activated anti-melanoma prodrugs.

alpha-Chymotrypsin, when immobilized in a collodion membrane, exhibits high activity and remarkable stability. When the immobilized proteinase is exposed to 15 mM ethyl N-acetyl-L-tyrosinate in dilute pH 8.5 buffer it generates a microenvironment which, indicator studies suggest, has an effective pH of approximately 4. The presence of this locally highly acidic region produces a marked increase in the rate of hydrolysis of BzPheal = Ala dissolved in the buffer solution (BzPheal = Ala is the acylhydrazide obtained from the reaction between N-benzoyl-L-phenylalaninal and N-acetyl-L-alanine hydrazide). The observed rate is 10-times greater than in comparable control experiments incorporating a concentrated buffer solution, in which a pH-gradient does not form. The enhanced hydrolysis rate is quantitatively explained if it is attributed to the approximately 20 microliters of pH 4 solution within the membrane. Other experimental data are also consistent with this hypothesis.