[3-Phosphono-2-imino-4-oxoimidazoline--a competitive substrate of AMP-dephosphorylating enzymes from the cytosol fraction of the rat heart].

3-Phosphono-2-imino-1-methyl-4-oxoimidazolidine (PIMOI), AMP and p-nitrophenyl phosphate (pNPP) were dephosphorylated in the presence of rat heart cytosol at 37 degrees C pH 6.3 at the rates of 0.71, 0.45 and 1.07 mumol/mg X h, respectively. When mixed together, these compounds inhibited the hydrolysis of each other, which points to the participation of common enzyme(s) in this process. The inhibitor of 5'-nucleotidase (alpha,beta-methylene)-ADP, did not affect PIMOI cleavage and moderately inhibited AMP hydrolysis (by ADP, did not affect PIMOI cleavage and moderately inhibited AMP hydrolysis (by 30-50%), thus suggesting that acidic phosphatases are responsible for PIMOI and AMP hydrolysis under these conditions (pH 6.3). Phosphocreatine (PCr) and phosphocyclocreatine (PcCr) were stable to hydrolysis by the cytosolic fraction. However, addition of AMP to the medium containing PCr or PcCr resulted in AMP phosphorylation down to ATP due to the effects of these phosphagens and, probably, of microcontaminations of ATP. This was followed by gradual disappearance of PCr or PcCr and by accumulation of Pi as a result of the "ATPase" activity in the cytosol. The hydrolysis of AMP, PIMOI and p-NPP was sensitive to sulfhydryl reagents [5,5'-dithio-bis-(2-nitrobenzoate) and, in part, 2,4-dinitro-fluorobenzene] and fluoride ion. Thus, PIMOI is a competitive substrate of acidic phosphatases in heart cytosol with respect to AMP and p-NPP. This may partly explain the protective effect of PIMOI on ischemic myocardium.