Fluorometric coupled enzyme assay for N-sulfotransferase activity of N-deacetylase/N-sulfotransferase (NDST).

N-Deacetylase/N-sulfotransferases (NDST) are critical enzymes in heparan sulfate (HS) biosynthesis. Radioactive labeling assays are the preferred methods to determine the N-sulfotransferase activity of NDST. In this study, we developed a fluorometric coupled enzyme assay that is suitable for the study of enzyme kinetics and inhibitory properties of drug candidates derived from a large-scale in silico screening targeting the sulfotransferase moiety of NDST1. The assay measures recombinant mouse NDST1 (mNDST1) sulfotransferase activity by employing its natural substrate adenosine 3'-phophoadenosine-5'-phosphosulfate (PAPS), a bacterial analog of desulphated human HS, Escherichia coli K5 capsular polysaccharide (K5), the fluorogenic substrate 4-methylumbelliferylsulfate, and a double mutant of rat phenol sulfotransferase SULT1A1 K56ER68G. Enzyme kinetic analysis of mNDST1 performed with the coupled assay under steady state conditions at pH 6.8 and 37 °C revealed Km (K5) 34.8 μM, Km (PAPS) 10.7 μM, Vmax (K5) 0.53 ± 0.13 nmol/min/μg enzyme, Vmax (PAPS) 0.69 ± 0.05 nmol/min/μg enzyme, and the specific enzyme activity of 394 pmol/min/μg enzyme. The pH optimum of mNDST1 is pH 8.2. Our data indicate that mNDST1 is specific for K5 substrate. Finally, we showed that the mNDST1 coupled assay can be utilized to assess potential enzyme inhibitors for drug development.