Catalyzes an amino-pyrimidine hydrolysis reaction at the C5' of the pyrimidine moiety of thiamine compounds to give a hydroxymethylpyrimidine (HMP). Displays low activity on 4-amino-5-aminomethyl-2-methylpyrimidine as substrate, indicating that the enzyme may act on a different HMP precursor that may derive from the human stomach food assumption or processing. Is probably involved in thiamine biosynthesis. Does not display thiaminase II activity, as it is unable to hydrolyze thiamine. 4-amino-5-aminomethyl-2-methylpyrimidine + H2O = 4-amino-5-hydroxymethyl-2-methylpyrimidine + NH4(+) kcat is 1.7 min(-1) for the hydrolysis of 4-amino-5-aminomethyl-2-methylpyrimidine. Activity is lost at pH 6. Cofactor biosynthesis; thiamine diphosphate biosynthesis. Homotetramer. H.pylori is expected to possess a reduced thiamine biosynthetic pathway. Some enzymes involved in de novo thiamine biosynthesis in other organisms, such as ThiO, ThiS and ThiG, are missing in this species. The very acidic environment of the stomach makes the accumulation of formylaminopyrimidine very unlikely because it is mainly a base-degraded derivative of thiamine; formylaminopyrimidine is the precursor identified in a thiamine salvage pathway in Bacillus species. Belongs to the TenA family.