Biosynthesis of PAPS in vitro by human liver. Measurement by two independent assay procedures

Replacing PAPS: In vitro phase II sulfation of steroids with the liver S9 fraction employing ATP and sodium sulfate

In vitro technologies provide the capacity to study drug metabolism where in vivo studies are precluded due to ethical or financial constraints. The metabolites generated by in vitro studies can assist anti-doping laboratories to develop protocols for the detection of novel substances that would otherwise evade routine screening efforts. In addition, professional bodies such as the Association of Official Racing Chemists (AORC) currently permit the use of in-vitro-derived reference materials for confirmation purposes providing additional impetus for the development of cost effective in vitro metabolism platforms. In this work, alternative conditions for in vitro phase II sulfation using human, equine or canine liver S9 fraction were developed, with adenosine triphosphate (ATP) and sodium sulfate in place of the expensive and unstable co-factor 3'-phosphoadenosine-5'-phosphosulfate (PAPS), and employed for the generation of six representative steroidal sulfates. Using these conditions, the equine in vitro phase II metabolism of the synthetic or so-called designer steroid furazadrol ([1',2']isoxazolo[4',5':2,3]-5α-androstan-17β-ol) was investigated, with ATP and Na₂ SO₄ providing comparable metabolism to reactions using PAPS. The major in vitro metabolites of furazadrol matched those observed in a previously reported equine in vivo study. Finally, the equine in vitro phase II metabolism of the synthetic steroid superdrol (methasterone, 17β-hydroxy-2α,17α-dimethyl-5α-androstan-3-one) was performed as a prediction of the in vivo metabolic profile.

Direct measurement and regulation of 3'-phosphoadenosine 5'-phosphosulfate (PAPS) generation in vitro

3'-Phosphoadenosine 5'-phospho[35S]sulfate (PAPS) biosynthesized from inorganic [35S]sulfate and ATP was separated from its radiolabeled precursor by reversed-phase paired-ion HPLC and quantified by on-line radiometric detection. This single-step procedure circumvents several problems inherent in conventional sulfotransferase-coupled assays employed in the measurement of PAPS formation. A good correlation was observed between the rate of PAPS generation assayed in several mammalian tissues measured by direct HPLC-radiometry and by coupling to the sulfation of minoxidil or 4-methylumbelliferone. Both AMP and ADP inhibited the rat liver sulfate-activating enzymes competitively with respect to MgATP2-, and the rate of PAPS production was decreased with decreasing ratios of [ATP]:[ADP] and [ATP]:[AMP]. It is possible that these adenine nucleotides regulate sulfate activation by kinetic control and by negative feedback modulation.

Biosynthesis of 3'-phosphoadenosine-5'-phosphosulfate (PAPS) in rat skin

A rapid and sensitive radiometric assay was developed for the measurement of 3'-phosphoadenosine-5'-phosphosulfate (PAPS) biosynthesis in rat skin extract. The formation of PAP35S from sodium 35sulfate and ATP was quantified by the transfer of the 35sulfate to minoxidil by rat liver minoxidil sulfotransferase (MST). The assay is sensitive enough for the detection of as little as 2 pmol of PAP35S. The PAPS-generating system showed a pH optimum of 8.6, with an apparent Km value of 1 mM for the ATP-Mg2+ complex and 68 microM for sodium 35sulfate. ATP and Mg2+, present individually or together in equimolar concentrations, were inhibitory above 8 mM. Excess (or free) ATP was a competitive inhibitor with respect to the ATP-Mg2+ complex; the apparent Ki measured was 0.32 mM. The specific activity of the PAPS-generating system, measured in rat skin cytosol was 0.15 nmol PAPS/min/mg protein. The importance of PAPS generation in detoxification and bioactivation of xenobiotics in skin is discussed.

Sulphate conjugation of minoxidil in rat skin

Minoxidil sulphotransferase (MST) activity was determined in the cytosolic fraction of rat skin and liver. MST of rat skin is similar to the P (phenol)-form of phenosulphotransferase (PST) of human tissues with respect to thermostability and inhibition by 2,6-dichloro-4-nitrophenol (DCNP). p-Nitrophenol, a prototype substrate of human P-PST form, inhibits MST at micromolar concentration while millimolar concentrations of dopamine and tyramine, substrates of human M-(monoamine)-PST, are required to elicit a similar degree of inhibition. The enzymatic transfer of 35S from sodium 35sulphate to minoxidil was also demonstrated suggesting that the rat skin is potentially capable of synthesizing 3'-phosphoadenosine-5'-phosphosulphate (PAPS) from inorganic sulphate and utilizing it for the biosynthesis of minoxidil sulphate, its active metabolite. Thus, it is conceivable that the pharmacological action of minoxidil as a promoter of hair growth could be carried out by the cutaneous tissues without the contribution of hepatic or other extrahepatic organs.