Griffiths (13) has presented evidence indicating that the browning reactions of banana fruit result from the enzymic oxidation of dopamine (3,4 dihydroxyphenylethyliamine) by polyphenoloxidase. Although dopamine does occur in various fruits and vegetables (23), it has not previously been implicated as an important substrate in browning reactions. The present investigation was undertaken to develop methods for preparation of banana polyphenoloxidase (PPO)3 and to determine some of its properties in comparison with phenol oxidases from other sources.

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Banana Polyphenoloxidase. Preparation and Properties

A phosphonate monoester, m-carboxyphenyl phenylacetamidomethylphosphonate, has been found to be a specific inhibitor of the class C beta-lactamase of Enterobacter cloacae P99. Inactivation is rapid (10(3) per second per molar concentration) and reactivation very slow (2.2 X 10(-6) per second). Apparently concerted with the inactivation, one equivalent (with respect to the enzyme) of m-hydroxybenzoate is released. Reactivation is accelerated by hydroxylamine and benzohydroxamate. This suggests that the loss of enzyme activity is due to phosphonylation of an active site functional group. This discovery holds the promise of a new general class of beta-lactamase inhibitors and, perhaps, antibiotics.

https://science.sciencemag.org/content/sci/246/4932/917.full.pdf

Inhibition of a class C beta-lactamase by a specific phosphonate monoester

Colorimetric assay for monoamine oxidase in tissues using peroxidase and 2,2'-azinodi(3-ethylbenzthiazoline-6-sulfonic acid) as chromogen

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(1) The inhibitor of γ-aminobutyrate transaminase (GABA-T), amino-oxyacetic acid (AOAA), drastically reduced the activity of GABA-T to 30 per cent of the control value, with a corresponding increase of brain GABA, but had no effect on the activity of glutamate decarboxylase (GAD)



(2) The monoamine oxidase (MAO) inhibitors phenelzine, phenylpropylhydrazine and phenylvalerylhydrazine, lowered GABA-T activity to 58, 49 and 48 per cent, respectively; this was associated with a marked elevation of brain GABA



(3) The action of phenelzine and phenylpropylhydrazine in vivo and in vitro could be abolished by pre-treatment of the tissue with the structurally related MAO inhibitors phenylisopropylhydrazine and trans-2-phenylcyclopropylamine These had no action on the GABA system in vivo, either on the GABA content or on the GABA-T activity These latter drugs, however, were unable to influence the effects of AOAA either on GABA or on GABA-T



(4) The possible mechanism of action on GABA and the enzyme activities of the GABA system is discussed

SOME EFFECTS OF MONOAMINE OXIDASE INHIBITORS ON THE METABOLISM OF γ-AMINOBUTYRIC ACID IN RAT BRAIN

Developmental lead exposure alters mitochondrial monoamine oxidase and synaptosomal catecholamine levels in rat brain

Michael, S. E. (1958). Ab8tr. Comm. 4thint,. Congr. Biochem., Vienna, no. 2-106, p. 28. Pedersen, K. 0. (1958). J. phys. Chem. 62, 1282. Popjaik, G. & McCarthy, E. F. (1946). Biochem. J. 40, 789. Ram, J. S. & Maurer, P. H. (1958). Arch. Biochem. Biophys. 76, 28. Reichmann, M. E. & Charlwood, P. A. (1954). Canad. J. Chem. 32, 1092. Schwert, G. W. (1957). J. Amer. chem. Soc. 79, 139. Smith, D. B., Wood, G. C. & Charlwood, P. A. (1956). Canad. J. Chem. 34, 364. Svedberg, T. & Pedersen, K. 0. (1940). The Ultracentrifuge. Oxford University Press. Warren, R. L. & Charlwood, P. A. (1953). Nature, Lond., 171, 353. Wieme, R. J. (1959). Clin. chim. Acta, 4, 317. Williams, J. W., van Holde, K. E., Baldwin, R. L. & Fujita, H. (1958). Chem. Rev. 58, 715.

/pdf/a-colorimetric-method-for-the-estimation-of-monoamine-w1n755813v.pdf

A colorimetric method for the estimation of monoamine oxidase.

Chymotrypsin, like most other enzymes with esterase activity, is inactivated by combination with organophosphates (Balls & Jansen, 1952). The inactive phosphorylated enzyme is relatively stable in water but hydroxylamine and picolinohydroxamic acid will restore the activity by nucleophilic displacement of the enzyme from the phosphoryl residue (Cunningham, 1954; Jandorf, Crowell & Levin, 1955). The only reported kinetic study of this process is by Cunningham (1954) on reactivation with hydroxylamine, but his conclusions as to mechanism are open to criticism as, at best, only about 30% of the original enzymic activity could be recovered. The present paper describes the complete reactivation of phosphorylated chymotrypsin by oximes and hydroxamic acids. The results necessitate some modification of views on the mechanisms for chymotryptic hydrolysis which have been proposed recently (Cunningham, 1957; Dixon & Neurath, 1957a; Davies & Green, 1958).

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A. L. Green

Papers

A colorimetric method for the estimation of monoamine oxidase.

The reactivation of phosphorylated chymotrypsin.