ABSTRACT
The mechanism by which bi-and trivalent cations affect human liver phosphatidate phosphohydrolase [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 Al[3+] and Cr[3+], however, was much lower being 2 mM and 1 mM, 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 Mg[2+] 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