Inhibition of Horse‐Liver Alcohol Dehydrogenase by Pt(CN) 4 2– and Au(CN) 2 –

Single and multiple‐inhibition experiments provide evidence that the complex anion Pt(CN) 4 2‐ is bound at a single site per subunit of liver alcohol dehydrogenase in strict competition with coenzyme, ADP‐ribose, AMP and (less than 100 mM) chloride ion, but independently of 1,10‐phenanthroline. Binding of adenosine decreases the stability constant for the enzyme · Pt(CN) 4 2‐ complex from 16 mM ‐1 to 4 mM ‐1 . Au(CN) 2 ‐ is bound at two sites per subunit of the enzyme. Both interactions are competitive with coenzyme, ADP‐ribose and AMP, but independent of 1,10‐phenanthroline. Only one site is available for interaction with Au(CN) 2 ‐ in the binary enzyme complexes formed with Pt(CN) 4 2‐ , chloride ion, or adenosine. Estimation of the corresponding binding constants for Au(CN) 2 ‐ gave evidence for cooperativity between the two Au(CN) 2 ‐ binding sites. These results strongly indicate that Pt(CN) 4 2‐ is bound at a site within the protein region occupied by the phosphate groups of the coenzyme, and it is suggested that arginine‐47 in the enzymatic amino‐acid sequence is part of a common binding site for negative charges on the coenzyme and coenzyme competitive anions such as Pt(CN) 4 2‐ , Au(CN) 2 ‐ , chloride and iodoacetate. The second binding site for Au(CN) 2 ‐ appears to be situated within the protein region occupied by the adenosine part of the coenzyme.