the relationship between cation-induced substrate configuration and enzymatic activity of phosphatidate phosphohydrolase from human liver

the mechanism by which bi-and trivalent cations affect human liver phosphatidatephosphohydrolase (pap) activity was investigated. bivalent cations up to 1 mm increased pap activity whereas at higher concentrations the activity of the enzyme decreased. the stimulatory concentration for trivalent cations such as al3+ and cr3+, however, was much lower being 2 m m and 1 m m, respectively. all cations affecting pap activity were also able to induce phase transition of phosphatidate from lamellar (la) to inverted hexagonal (hii) form. the rate of la-hii transition was different for each cation. at 100 mm concentration of mg2+ only 26% of the original phosphatidate remained in la form and for other cations tested ranged from 14.5% to 76%. the phase transition was blocked by edta. magnesium from 0.8 to 1.5 mm concentration raised pap activity (3-fold) with la form of substrate but not with the hii phase. monovalent cations such as na+ and k+ neither affected enzyme activity nor substrate configuration. these data suggest that cation-induced pap activation is not as a result of cation-protein interaction, but is due to formation of a suitable substrate configuration for the enzyme catalysis during phosphatidate phase transition. it appears that the real substrate configuration for pap activity is situated between la and hii phases.