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.