Phenolsulphotransferase activity in the developing mosquito (Aedes togoi)

Acetyl CoA generation and N-acetylation of serotonin (5HT) in the mosquito, Aedes togoi

Acetyl CoA synthetase (ACS; EC 6.2.1.1) was studied in the mosquito, Aedes togoi, by a novel assay which coupled the acetyl-CoA generated to p-aminosalicylic acid (ASA). The N-acetylated product was determined by an HPLC-fluorimetric procedure. High ACS activity was observed in the newly-pupated pupae of both sexes and in the adult male mosquito whose activity was five times that of the female. Acetyl CoA-dependent N-acetyltransferase (NAT; EC 2.3.1.5) activity toward serotonin (5HT) was also studied using HPLC-electrochemical detection (HPLC-ECD). A progressive increase in the 5HT-NAT activity was observed from the fourth-instar larvae to the adult mosquito with the latter showing 6-fold higher activity in the head compared to the abdomen-thorax region. Kinetic studies on the pupal enzyme extracts showed that the apparent Km values for 5HT and acetyl CoA were 63 and 66 microM respectively. Tryptamine inhibited 5HT-NAT non-competitively with a Ki value of 8 microM.

Sulphate conjugation of serotonin and N-acetylserotonin in the mosquito, Aedes togoi

The sulphate conjugation of serotonin and N-acetylserotonin (NAS) was studied in the mosquito, Aedes togoi, using a high-pressure liquid chromatography-radioisotopic procedure. This involved the transfer of the sulphate group from 3'-phosphoadenosine-5'-phosphosulphate (PAP35S) to serotonin or NAS by phenolsulphotransferase (PST). NAS, the acetylated product, is less polar than serotonin but it is a better substrate of PST. Its removal would conceivably be facilitated by sulphate conjugation which renders it more water-soluble. This sequential two-step reaction, comprising N-acetylation of serotonin and sulphate conjugation of the acetylated product (NAS) was also demonstrated in vitro. The developmental profile of PST activity using NAS as substrate showed that peak activity occurred at pupation and increased progressively for a few days after emergence of the adult. Based on the selective inhibition of the "P" form of PST by 2,6-dichloro-4-nitrophenol (DCNP), the sulphate conjugation of NAS appeared to be catalysed by the "M" form of PST.

An HPLC-ECD procedure for measuring total phenolsulfotransferase (PST) activity in human liver, platelets and blood

The phenolsulfotransferase (PST) activity in human liver, platelets and blood was measured under saturating concentrations of the conjugating agent, 3'-phosphoadenosine-5'-phosphosulfate (PAPS). Conventional PST assays employ PAP35S at suboptimal concentrations. In addition, the sulfate conjugate formed, namely N-acetyldopamine-sulfate (NADA-sulfate) was quantified directly by high-pressure liquid chromatography cum electrochemical detection (HPLC-ECD). NADA, the biogenic amine acceptor used in this study appeared from kinetic data to be a substrate of both the P and M forms of PST when used in micromolar concentration. Two apparent Km values of 4.2 mumol/l and 22.6 mumol/l were observed. In contrast, only one apparent Km value was evident when the assay was carried out in the presence of 2,6-dichloro-4-nitrophenol (DCNP), a selective inhibitor of the P form of PST or after heat treatment under specified conditions which inactivates the M form of PST. Thus measurement of PST activity with NADA as the acceptor substrate permits the determination of total PST activity and a parallel assay with the inclusion of DCNP would distinguish the two variants of PST, both of which appear to be present in all human tissues.

Sulphate conjugation in human liver. Direct measurement of N-acetyldopamine-sulphate

The overall three-step sulphate conjugating activity of the human liver was determined by the direct measurement of N-acetyldopamine (NADA)-sulphate from ATP and inorganic sulphate. NADA-sulphate was separated from NADA and quantified by HPLC-ECD (high-pressure liquid chromatography-electrochemical detection). The overall sulphate conjugation of NADA showed at pH optimum of 8.0 with apparent Km values for sodium sulphate and NADA of 103 microM and 4.8 microM. respectively. The optimum concentration of ATP was 5 mM and that for Mg2+ ions was 7 mM. The specific activities of 17 samples of human liver, measured by this HPLC-ECD procedure ranged from 940 to 23,128 pmol NADA-sulphate/hr/mg protein. A comparison of these values with those determined by the radiometric method developed previously showed a direct correlation coefficient, r = 0.89. The rates of overall sulphate conjugation of NADA and dopamine measured in these samples by the radiometric assay procedure were also significantly correlated with r = 0.82.

Kinetics of sulphate conjugation in extracts of human foetal liver cells in culture

Sulphate conjugation was investigated using extracts of human foetal liver cells in culture. The three reactions which are involved in sulphate conjugation were measured singly or in combination: they are (i) the PAPS generation catalyzed by ATP-sulphurylase and adenosine 5'-phosphosulphate (APS)-kinase, (ii) the phenolsulphotransferase (PST) reaction, and (iii) the overall sulphate conjugation which comprises the above three reactions. All were radiometric assays employing PAP35S or sodium 35sulphate. N-acetyldopamine (NADA) was the substrate of choice although the reactions were also demonstrated with dopamine and 1-naphthol. Kinetic studies with NADA showed two pH optima of 6.7 and 8.6 for the overall sulphate conjugation and the PST reaction while the PAPS generation occurred maximally at pH 8.0. The apparent Km value for NADA measured by both the PST and the overall sulphate conjugation reactions was the same, being 38 microM, while that for inorganic sulphate, of 107 microM and 240 microM (measured by the overall sulphate conjugation reactions and by PAPS generation, respectively) was two orders of magnitude higher than that of PAPS, which was 2.57 microM. It was possible to maintain a relatively constant level of the three activities of sulphate conjugation in confluent, quiescent cultures. The importance of sulphate conjugation for detoxification in foetal cells is discussed.

Does PAPS generation determine the overall sulfate conjugation in human platelets?

The formation of 3'-phospho-adenosine-5'phosphosulfate (PAPS) depends on essentially two enzymic reactions catalysed by ATP-sulfurylase and APS-kinase (adenosine 5'-phosphosulfate-kinase). In this paper, PAPS generation by human platelets was determined by the transfer of 35sulfate from PAP35S formed in vitro to N-acetyldopamine (NADA), using the phenolsulfotransferase extracted from rat liver. A pre-requisite of this quantitative procedure was the prior inhibition of the sulfate-activating system in the latter enzyme preparation. This was accomplished by the addition of 10 mM EDTA and 14 mM pyrophosphate. The PAPS-generating system of human platelets exhibited two pH peaks with higher activity at pH 8 than pH 6. Optimal concentrations of ATP and Mg++ at 7 mM were required for the two reactions. PAPS generation so measured showed a highly significant correlation with the overall sulfate conjugation of NADA: a correlation coefficient of 0.96 was established from data obtained from 60 platelet preparations of normal subjects.