Involved in purine and pyrimidine breakdown rather than in pyrimidine salvage, especially in response to dark stress (PubMed:19293370, PubMed:21235647, PubMed:21599668, PubMed:30787180). Together with URH2, required for efficient inosine and xanthosine hydrolytic activities (PubMed:21599668, PubMed:30787180). Unable to use cytidine as a substrate (PubMed:19293370). Can use uridine, inosine, adenosine as well as the cytokinin derivative isopentenyladenine-riboside as substrates (PubMed:19293370). Hydrolyzes also xanthosine with high efficiency (PubMed:21235647). H2O + uridine = D-ribose + uracil H2O + xanthosine = D-ribose + xanthine H2O + inosine = D-ribose + hypoxanthine adenosine + H2O = adenine + D-ribose kcat is 2.65 sec(-1) with xanthosine as substrate (PubMed:30787180). kcat is 33.5 sec(-1) with uridine as substrate (PubMed:30787180). kcat is 23.3 sec(-1) with xanthosine as substrate (when in complex with URH2) (PubMed:30787180). kcat is 55.3 sec(-1) with uridine as substrate (when in complex with URH2) (PubMed:30787180). kcat is 42.3 sec(-1) with inosine as substrate (when in complex with URH2) (PubMed:30787180). Homodimer (PubMed:30787180). Component of the NSH heterocomplex made of URH1/NSH1 and URH2/NSH2 which exhibits strong xanthosine nucleosidase activity (PubMed:30787180). Interacts with URH2 (PubMed:30787180). Expressed ubiquitously in leaves, flowers, stems, pollen cells, root tip meristem and root vasculature. Normal seedling germination and plant growth and development in standard conditions, despite an abnormal accumulation in the roots of uridine and of other pyrimidine metabolites as well as of xanthosine, but no accumulation of inosine (PubMed:21599668). Accelerated senescence accompanied by marked accumulation of uridine and xanthosine under conditions of prolonged darkness leading to pale green/yellow plants due to increased chlorophyll degradation (PubMed:21235647). The roots of the double mutant urh1 urh2 accumulates strong levels of xanthosine (PubMed:21599668, PubMed:30787180). Belongs to the IUNH family.