By using cytoplasmic and mitochondrial serine transhydroxymethylase isoenzymes from rabbit liver, it was shown that both enzymes exhibited similar ratios of serine transhydroxymethylase/threonine aldolase activities. Both enzymes catalysed the removal of the pro-S hydrogen atom of glycine, which was greatly enhanced by the presence of tetrahydrofolate. The cytoplasmic as well as the mitochondrial enzyme catalysed the synthesis of serine from glycine and [3H2]formaldehyde in the absence of tetrahydrofolate. The results are consistent with our previous suggestion that a role of tetrahydrofolate in the serine transhydroxymethylase reaction is to transport formaldehyde in and out of the active site (Jordan & Akhtar, 1970). The isoenzymes, however, showed remarkable differences in their inactivation by inhibitors. The serine transhydroxymethylase as well as the threonine aldolase activities of the cytoplasmic enzyme were inactivated in a similar fashion by chloroacetaldehyde, iodoacetamide, bromopyruvate and glycidaldehyde (2,3-epoxypropionaldehyde). These inhibitors had no effect on the two activities of the mitochondrial enzyme. The rate of inactivation of the cytoplasmic enzyme by glycidaldehyde was enhanced by the presence of glycine but decreased by the presence of serine. The implications of these results to the mechanism of catalysis and the nature of the active site of the enzymes are discussed.
A new, simple and low cost spectrophotometric method for the determination of methyldopa in pharmaceutical preparations was developed. The method was based on the coupling of methyldopa with 2,6-dichloroquinone-4-chlorimide (DCQ). The absorbance maximum (λ max) of the resulted colored product was at 400 nm. Different buffers were used to determine the optimal pH for the reaction. About 1% w/v acetate buffer with pH 8.0 gave the optimal pH required for the reaction. Of the different solvents tried, water and ethanol were found to be the most suitable solvents. Beer's law was obeyed in concentration range of 4-20 μg/ml methyldopa. The correlation coefficient was found to be (r = 0.9975). The limit of detection and limit of quantification were 1.1 μg/ml and 3.21 μg/ml, respectively. The reaction ratio between methyldopa and DCQ was studied and found to be 1:3. The work included the study of the possible interference of hydrochlorothiazide found in combination with methyldopa tablets. The method was validated and results obtained for the assay of two different brands of methyldopa tablets were compared with the BP method (colorimetric). The repeatability and reproducibility of the developed method were evaluated and the obtained results quoted. The derivative formed as a result of the reaction of methyldopa with DCQ was isolated and its possible mechanistic pathway was suggested.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.