The enzymic oxidation of chlorogenic acid and some reactions of the quinone produced

o-Quinones formed in plant extracts. Their reaction with bovine serum albumin

1. The reactions between chlorogenoquinone, the o-quinone formed during the oxidation of chlorogenic acid, and bovine serum albumin depend on the ratio of reactants. 2. When the serum albumin is in excess, oxygen is not absorbed and the products are colourless. This reaction probably involves the thiol group of bovine serum albumin; it does not occur with bovine serum albumin which has been treated with p-chloromercuribenzoate, iodoacetamide or Ellman's reagent. 3. When bovine serum albumin reacts with excess of chlorogenoquinone, oxygen is absorbed and the products are red. The red colour is probably formed by reaction of the lysine in-amino groups of bovine serum albumin, as it is prevented by treating the protein with formaldehyde, succinic anhydride or O-methylisourea. 4. Bovine serum albumin modified by a 1.5-fold (BSA-Q) and a fivefold (BSA-Q2) excess of chlorogenoquinone were separated by chromatography on DEAE-Sephadex A-50, and some of their properties observed. 5. Reaction of BSA-Q2 with fluorodinitrobenzene suggests that the terminal alpha-amino group, as well as lysine in-amino groups, are combined with chlorogenoquinone.

o-Quinones formed in plant extracts. Their reactions with amino acids and peptides

1. The reactions of amino acids and peptides with the o-quinones produced by the enzymic oxidation of chlorogenic acid and caffeic acid have been studied manometrically and spectrophotometrically. 2. Amino acids, except lysine and cysteine, react primarily through their alpha-amino groups to give red or brown products. These reactions, which compete with the polymerization of the quinones, are followed by secondary reactions that may absorb oxygen and give products with other colours. 3. The in-amino group of lysine reacts with the o-quinones in a similar way. The thiol group of cysteine reacts with the quinones, without absorbing oxygen, giving colourless products. 4. Peptides containing cysteine react with the o-quinones through their thiol group. 5. Other peptides, such as glycyl-leucine and leucylglycine, react primarily through their alpha-amino group and the overall reaction resembles that of the N-terminal amino acid except that it is quicker. 6. With some peptides, the secondary reactions differ from those that occur between the o-quinones and the N-terminal amino acids. The colours produced from carnosine resemble those produced from histidine rather than those from beta-alanine, and the reactions of prolylalanine with o-quinones are more complex than those of proline.

Reaction of tyrosine oxidation products with proteins of the lens

Oxidation of tyrosine in the presence of bovine lens proteins leads to the formation of brown or black melanoproteins. Both tyrosinase and the oxidizing system of ferrous sulphate-ascorbic acid-EDTA are effective. The fluorescence of the lens proteins is both altered and enhanced by the tyrosine-oxidizing systems. Their fluorescence spectra resemble those of urea-insoluble proteins of human cataractous lens and of 1,2-naphthaquinone-proteins of naphthalene cataract. The lens proteins lose their thiol groups and, in acid hydrolysates of treated beta-and gamma-crystallins, a substance has been detected chromatographically that behaves similarly to a compound formed when 3,4-dihydroxyphenylalanine (dopa) is oxidized by tyrosinase in the presence of cysteine. Analysis and behaviour of this substance from hydrolysates of lens proteins suggest that it is a compound of cysteine and dopa.

l-Phenylalanine Ammonia-Lyase Activity During Germination of Phaseolus vulgaris

l-Phenylalanine ammonia-lyase (PAL) activity develops in excised bean axes after approximately 5 hours of incubation and reaches a maximum level after 14 hours of incubation. Light does not affect the development of activity, but puromycin, cycloheximide, actinomycin D, and 5-fluorouracil inhibit.During this period of incubation both d- and l-p-fluorophenylalanine stimulate fresh weight increase and both inhibit the development of PAL activity. Neither l- nor d-phenylalanine stimulates fresh weight increase while the former inhibits development of PAL activity and the latter has no effect. Neither d isomer is deaminated while l-p-fluorophenylalanine is deaminated at about one-half the rate of l-phenylalanine. It is suggested that fluorophenylalanine does not stimulate the fresh weight increase by its effects on the phenolics pathway.Trans-cinnamic acid was found to inhibit both the development of PAL activity and the in vitro deamination of l-phenylalanine. Various hydroxycinnamic acids, although inhibiting the development of PAL activity, had little or no effect on the in vitro deamination of l-phenylalanine.No tyrosine ammonia-lyase activity was found in the axes and l-tyrosine had no effect on the in vitro deamination of l-phenylalanine.The pattern of PAL development in intact seedlings differs markedly from that which occurs in the excised axes, although light also has no effect on the course of activity.

Inhibition of banana polyphenoloxidase by 2-mercaptobenzothiazole

2-Mercaptobenzothiazole is an exceptionally potent inhibitor of banana polyphenoloxidase; it significantly delays the onset of substrate oxidation at concentrations as low as 10(-7)M and causes prolonged inhibition at 2 x 10(-5)M or higher. Inhibition results from formation of a dissociable, mixed complex between the enzyme and the inhibitor.