Histamine exists predominantly as the NT-H tautomer of the monocation (IIa) at a physiological pH of 7.4 and structure-activity studies indicate that this tautomer is likely to be the pharmacologically active species for both HI and HP receptors. Effective Hz-receptor agonists appear to require a prototropic tautomeric system whereas HI-receptor agonists do not need to be tautomeric. This identifies a chemical difference in the receptor ...
