Electrochemical determination of diphenol oxidase activity using high-pressure liquid chromatography

Effect of prophenoloxidase expression knockout on the melanization of microfilariae in the mosquito Armigeres subalbatus

Melanization is an effective defence reaction used by mosquito hosts to kill malarial and filarial worm parasites. Although phenoloxidase (PO) has long been considered to be the key enzyme in the biosynthesis of melanotic material in insects, there is no direct evidence verifying its role in parasite melanization. To elucidate the role of PO in the melanization of microfilariae (mf) by mosquitoes, a double subgenomic Sindbis (dsSIN) recombinant virus was used to transduce Armigeres subalbatus mosquitoes with a 600 base antisense RNA targeted to the highly conserved copper-binding region of an Ar. subalbatus PO gene. Compared with controls, haemolymph PO activity in mosquitoes transduced with antisense RNA was significantly reduced. When these mosquitoes were challenged with Dirofilaria immitis mf, the melanization of mf was almost completely inhibited. These data verify that PO is an essential component of the biochemical pathway required for the melanization of parasites, and that the dsSIN expression system represents a useful tool in the functional analysis of endogenous gene expression in mosquitoes.

Polyphenol oxidase activity staining in polyacrylamide electrophoresis gels

An analytical method allowing the detection of polyphenol oxidase activity on polyacrylamide gel electrophoresis (PAGE) is described. The method is rapid, sensitive and specific and is based on a coupling reaction between 4-tert-butyl-o-benzoquinone and the aromatic amine, 4-amino-N,N-diethylaniline sulphate. Catecholase activity of polyphenol oxidase appears as blue stained bands on a colourless background.

High-performance liquid chromatography-based assays of enzyme activities

Interest in using HPLC to assay enzymatic reactions continues to grow as evidenced by the more than 100 papers published during the early 1990s. HPLC can be used for any enzymatic assay that requires separation of substrates and products before quantifying the extent of the reaction. The popularity of HPLC-based assays is due to several reasons: (1) HPLC offers unsurpassed precision, specificity, sensitivity, and reproducibility. (2) Powerful microcomputers and user-friendly software automate the running of samples and collection and processing of data. (3) Current columns, especially C18 packings, separate a very wide variety of samples, and (4) A variety of on-line detectors provide a means to detect virtually any compound. This review surveys recent papers on the development of HPLC-based assays for enzymes that degrade or otherwise modify macromolecules. Methods for assaying enzymes involved in metabolic pathways are also reviewed. Work by the authors in developing HPLC-based assays for mitochondrial enzymes that use GTP/GDP and other nucleotides that cannot be or are not easily assayed by enzyme-coupled assays is discussed. These enzymes include nucleoside diphosphate kinase, succinate thiokinase, and GTP-AMP phosphotransferase. The assays are suitable for determining the submitochondrial compartmentation of enzyme activities. Finally, current and anticipated trends in HPLC technology, including new column packings and the trend toward smaller columns that give faster separations, are reviewed in relation to enzyme assays.

A stopped spectrophotometric assay for the dopa oxidase activity of tyrosinase

A stopped spectrophotometric assay for the dopa oxidase activity of tyrosinase has been developed to enable large numbers of samples to be screened very rapidly. The assay measures the pink pigment formed by the reaction of Besthorn's hydrazone (3-methyl-2-benzothiazoninone hydrazone, or MBTH) with dopaquinone, the product of oxidation of L-dopa by tyrosinase. Addition of perchloric acid stops the reaction and precipitates protein, enabling turbid as well as non-turbid samples to be assayed. Stability of the pink product is enhanced in acid solution and the pigment has a sharp absorbance maximum at 505 nm such that it is easily measured spectrophotometrically. Using the stopped assay, tyrosinase is detectable only in mammalian cell lines expected to express the enzyme, and the specificity of the assay has also been confirmed using tyrosinase inhibitors. The stopped MBTH assay is approx. 15-times more sensitive than the widely used dopachrome assay and can reliably detect the formation of as little as 350 pmol of product.

Relationship of hemolymph phenol oxidase and mosquito age in Aedes aegypti

Monophenol oxidase (MPO) and diphenol oxidase (DPO) activity in hemocytes and cell-free plasma perfused from 7-, 14-, 21-, and 28-day-old Aedes aegypti mosquitoes were compared. A progressive decrease of enzyme activity was detected as mosquito age increased, and this decrease was significant in both hemocytes and cell-free plasma when mosquitoes were 28 days old as compared with that found in 7-day-old mosquitoes. There was no significant difference in total hemolymph protein as mosquito age increased. Although this decreased MPO and DPO activity might be partially responsible for the reduced immune response against filarial worms previously reported for older mosquitoes, other factors undoubtedly play a significant role.

A continuous spectrophotometric method for the determination of diphenolase activity of tyrosinase using 3,4-dihydroxymandelic acid

A continuous spectrophotometric method for the rapid determination of diphenolase activity of tyrosinase is described. It uses 3,4-dihydroxymandelic acid (DOMA) as the substrate of tyrosinase and measures the final product, 3,4-dihydroxybenzaldehyde (DOBA). The spectrum of this product shows a bathochromic displacement of its absorbance maximum when the pH increases. The optimization of the method is described by using tyrosinase from several biological sources, whose enzymatic activities show different optimal pH. Thus, the enzymatic activity of mushroom tyrosinase was assayed at pH 7.5 and monitored at 350 nm (epsilon 350 pH 7.5 (DOBA) = 15,200 M-1 cm-1), whereas the spectrophotometric experiments with grape tyrosinase were carried out at pH 3.0 and monitored at 310 nm (epsilon 310 pH 3.0 (DOBA) = 9200 M-1 cm-1). The method for mushroom tyrosinase was found to be 50-fold more sensitive than the commonly used dopachrome assay, whereas for grape tyrosinase the method was found to be threefold more sensitive than the commonly used o-quinone production assay. The great solubility and stability of the chromophoric product, DOBA, as well as its high molar absorptivities at any pH, enable the method to be employed to determine the diphenolase activity of tyrosinase from different biological sources.

Oxidative decarboxylation of 3,4-dihydroxymandelic acid to 3,4-dihydroxybenzaldehyde: electrochemical and HPLC analysis of the reaction mechanism

Cyclic voltammetric and chronoamperometric data are consistent with a process in which 3,4-dihydroxymandelic acid (DOMA) is oxidized initially in a two-electron step to its corresponding o-benzoquinone. This species is unstable and undergoes the rate-determining loss of CO2 (k = 1.6 s-1 at pH 6 and 25 degrees C) to give an unobserved p-benzoquinone methide intermediate that rapidly isomerizes to 3,4-dihydroxybenzaldehyde (DOBAL), DOBAL is also electroactive at the applied potential and is oxidized in a two-electron step to 4-formyl-1,2-benzoquinone. Subsequent reactions of 4-formyl-1,2-benzoquinone include the oxidation of unreacted DOMA and the hydration of its aldehyde functional group. Oxidation of DOMA directly to its p-benzoquinone methide apparently does not occur. Derivatives of mandelic acid (e.g., 4-hydroxymandelic acid) that are expected to give only their corresponding p-benzoquinone methides upon oxidation afford redox behavior that differs distinctly from that for DOMA.