John E. Teggins

The reactivities of isomers of dichlorodiammine-platinum (II) with dehydrogenase enzymes. Evidence for inhibition via cross-linkage.

Abstract The reversible inhibition of malate dehydrogenase ( l -malate:NAD+ oxidoreductase, EC 1.1.1.37), lactate dehydrogenase ( l -lactate:NAD+ oxidoreductase, EC 1.1.1.27) and horse liver alcohol dehydrogenase (alcohol:NAD+ oxidoreductase, EC 1.1.1.1) by both cis- and trans-dichlorodiammine-platinum (II) (Pt(NH3)2Cl2) were carried out at pH 7.1 and 25°C. Inhibition of both liver and yeast alcohol dehydrogenase were measured at 4°C due to the latters instability at higher temperature for long periods of time under the experimental conditions used in this study. The equilibrium constant (Ke) was calculated for each enzyme-platinum complex system. It was shown that inhibition of both malate dehydrogenase and liver alcohol dehydrogenase was independent of the particular platinum isomer, while the trans isomer was a significantly better inhibitor than the cis form when either yeast alcohol dehydrogenase or lactate dehydrogenase was used. Thus, it has been proposed that the two former enzymes are being inhibited by a monodentate chelation with the platinum derivatives while the latter enzymes are being inhibited by a bidendate chelation. It has also been proposed that absolute differences in inhibition of various enzymes by a specific platinum inhibitor is due to different goemetries about the inhibitor site while similar inhibition values are caused by similar geometries.

The blocking of the tetrachloroplatinate (II) inhibition of malate dehydrogenase by sulfur-containing amino acids

Abstract The inhibition of malate dehydrogenase ( l -malate:NAD oxidoreductase, EC 1.1.1.37) by the tetrachloroplatinate (II) complex, PtCl42−, in the presence of various concentrations of the amino acids d l -methionine and l -cysteine was measured. The relative concentrations of PtCl42− to malate dehydrogenase was 100:1. From the data, the half-life and the relative rate of inhibition in the presence and absence of the amino acids were calculated. Using these values it was possible to calculate a stability constant for each system. The stability constants for the malate dehydrogenase-PtCl42−- l -cysteine ( K c ), malate dehydrogenase-PtCl42−- d l -methionine ( K m ) and, malate dehydrogenase-PtCl42−( K E ) were 56 M−1, 917 M−1 and 8·105 M−1, respectively. The reversibility of the malate dehydrogenase-PtCl42− complex was also demonstrated, by addition of d l -methionine to the completely inhibited enzyme. About 40% of the enzyme activity was regenerated. Using the stability constants it was calculated that 27% of the enzymes activity should be regenerated. From the results it is suggested that the free platinum complexes may be maintained in solution for a longer period of time if some or all of the halide ligands were replaced by sulfur groups from molecules like cysteine or methionine.

The inhibition of malate dehydrogenase by chlorammine-platinum complexes

Abstract Equilibrium association constants have been calculated for various platinum (II) and platinum (IV) complexes. The association constants were greatest for the dinegatively charged state, regardless of the valence state of the platinum, and the constant decreased considerably as the charge increased. There were no measurable values for positively charged complex states. The conclusion is that the electrostatic charge of the platinum complex is the most important factor causing the inhibition of the enzyme, and the steric differences have only a minor effect, while geometric variation as in the comparison of cis and trans dichlorodiamine-platinum (II) isomers yields no differences in inhibition.

Interactions of cis- and trans-platinum(II) complexes with dehydrogenase enzymes in the presence of different mono- and polynucleotides: evidence for a ternary complex.

The inhibition of several dehydrogenase enzymes by cis- and trans-Pt(NH3)2Cl2 have been measured in the presence of baker yeast ribonucleic acid (RNA), calf thymus and salmon sperm deoxyribonuclic acid (DNA) and several mononucleotides (AMP and ATP). The binding constants for the interaction of the platinum complexes to the nucleotides have been calculated and a comparison of those values to the previously calculated platinum complex-enzyme binding constants strongly suggest that platinum compounds are more tightly bound to the enzymes. The binding of the platinum complexes to most of the enzymes was decreased in the presence of any nucleotide, yet it was observed that when using rabbit muscle (M4) lactate dehydrogenase the mononucleotides reduced the binding to a lesser degree while the polynucleotides actually enhanced the platinum-enzyme interaction. The implications of these interactions are discussed.

KINETICS OF THE CIS-DICHLORODIAMMINEPLATINUM(II)-OXALATE REACTION IN AQUEOUS SOLUTION

Abstract The substitution of the chloride ligands in cis-Pt(NH3)2Cl2 by oxalate has been studied in neutral and slightly acidic aqueous solutions. At 50°, the reaction is first-order with respect to complex concentration. The estimated first-order rate constant (3.9 × 10−4sec−1 at 50°) and the activation energy (17 kcals/mole) are similar to the corresponding values reported for the aquation of the complex ion. The reaction rate is only slightly pH dependent in the pH range 3–7 indicating that C2O4 −2 and HC2O4 − have similar reactivities. The rate of the reaction is inhibited by the presence of excess free chloride. These data have been interpreted in terms of a mechanism involving the rate-determining aquation of the complex. By comparison with previous studies in which amine molecules were employed as substituting groups oxalate is a very weak nucleophile. It is suggested that the known biological activity of dichloro complexes of platinum(II) involves reaction at nitrogen-containing rather than oxygen...

STABILITIES OF IRON(III) COMPLEXES WITH 3- AND 4-HYDROXYPYRIDINE

Abstract Equilibrium constants for the reaction of 3- or 4-hydroxypyridine with iron(III) were measured in aqueous solution. The experimental method involved the measurement of hydrogen ion concentration and optical absorbance for each of a series of reaction mixtures. The results are consistent with the interpretation that the separate interactions between iron(III) and the hydroxypyridines are of the hard acid-hard base type, with sigma bonding alone being important. Equilibrium constants are reported for both the reactions in which iron(III) displaces a proton in protonated hydroxypyridine and for the direct combination of iron(III) and the hydroxypyridine.

Stability of the iron(III) complex with 2-hydroxypyridine

Abstract The formation of the complex formed between 2-hydroxypyridine and ferric ion has been studied in weakly acidic aqueous solution. Prior to the acquisition of the data for this equilibrium study it was necessary to study the equilibrium for the protonation of 2-hydroxypyridine in aqueous solution. A pK value of 0.98 was obtained for the protonation reaction at unit ionic strength at 25°. The formation constant for the 1:1 complex formed by the displacement of hydrogen ion by ferric ion in 2-hydroxypyridine at 25° and unit ionic strength was found to be 5.6. This value is much higher than the corresponding constants for 3- and 4-hydroproxypyridines. This evidence strongly suggests that interaction between the ferric ion and both the oxygen and nitrogen centers in 2-hydroxypyridine enhances complex stability.