Weak electrophile selective characteristics of the rat preneoplastic marker enzyme glutathione S-transferase P-form, GST-P (7-7): a theory of linear free energy relationships for evaluation of the active site hydrophobicity of isoenzymes
Subsite (the H-site) hydrophobicity of the rat glutathione S-transferase P-form (GST-P, 7-7, pi class species) and six other GST species of the alpha and mu classes was examined theoretically and experimentally by application of linear free energy relationships (LFERs) with a series of S-alkylated glutathiones, GS(CH2)n-1CH3 (n = 1-12). Plots of log Ki (inhibition constant) versus n were used to generate LFERs for microscopic hydrophobic interactions. The free enthalpic change per methylene group (-deltadeltaG degrees, absolute value) evaluated for GST-P (1.8 kJ/mol) was lower than those for the other six forms (2.4-3.5 kJ/mol). In addition, the enthalpic change (deltadeltaH degrees) determined from van't Hoff plots was much smaller for GST-P (0.5 kJ/mol) than the GST1-1 value (5.9 kJ/mol). As these thermodynamic parameters, deltadeltaG degrees and deltadeltaH degrees, may be considered as indirect and direct measures of GST hydrophobicity respectively, the H-site hydrophobicity of GST-P is thus very low as compared with those of other forms, clearly indicating that the pi class GST-P selectively targets for weak electrophiles, i.e. water-soluble carcinogens such as acrolein and hydroxyalkenals. The finding also defines a host-defensive role of the preneoplastic cells against the carcinogenic insult, although paradoxical.