Nicotinate hydroxylase, part of the hnx cluster involved in the purine degradation (PubMed:4581274). The nicotinate hydroxylase hnxS accepts nicotinate as a substrate and catalyzes the first step of nicotinate catabolism (PubMed:4581274). HnxS accepts also hypoxanthine, but not xanthine, as a substrate (PubMed:4581274, PubMed:363427, PubMed:29212709). The major facilitator-type transporters hxnP and hxnZ are probably involved in the uptake of nicotinate-derived metabolites, and the oxidoreductases hxnT and hxnY in the further metabolism of 6-OH nicotinic acid (PubMed:4581274). Binds 2 [2Fe-2S] clusters. Binds 1 Mo-molybdopterin (Mo-MPT) cofactor per subunit. Allopurinol inhibits catalytic activity in a linear fashion (PubMed:363427). Optimum pH is 9.4. Expression is induced by nicotinate and 6-OH nicotinate, subject to nitrogen metabolite repression mediated by the GATA factor areA, and strictly regulated by the cluster-specific transcription regulator hnxR (PubMed:4581274, PubMed:29212709). Belongs to the xanthine dehydrogenase family.