Activity of RNase from porcine pancreas was enhanced by treatment with Pb~(2+) at low concentration (0.17×10~(-2)～0.6×10~(-2)μmol/L) but was indibited by Pb~(2+) at high concentration (over 0.85×10~(-2) μmol/L). Pb~(2+) at the high concentration could competitively displace Ca~(2+) from RNase and the fluorescence titration showed that one molecule of RNase had three binding sites for Pb~(2+), the association constant κ_(siv for its low-affinity Pb~(2+) -binding site was 6.03×10~6(mol/L)~(-1). The extended X-ray absorption fine structure (EXAFS) spectrum demonstrated that Pb~(2+) coordinated nitrogen or oxygen atoms of the active site of RNase or other amino acid residue of RNase, Pb-N (or O) bond length was 0.242nm and o.312nm, and coordination number was 2, respectively. The secondary structure of RNase was greatly damaged by Pb~(2+) at high concentration resulting in the decrease of α-helix, β-sheet andβ-turn contents and the increase of random coil contents.

Key words: RIBONUCLEIC ACID, ACTIVITY, COORDINATION, ENVIRONMENT, ENZYME, STRUCTURE, LEAD, SPECTROGRAPHIC ANALYSIS, HEAVY METAL POLLUTION, PHYSIOLOGY