Copper-specific chelators as synergists to herbicides: 1. Amphiphilic dithiocarbamates, synthesis, transport through lipid bilayers, and inhibition of Cu/Zn superoxide dismutase activity

Controlled inhibition of the enzyme Cu/Zn superoxide dismutase (SOD), one of the most important enzymes of the antioxidant defense system of aerobic organisms, is the subject of this paper. It is desirable in several noted medical and agricultural applications. This paper describes the synthesis of a very limited sublibrary of bifunctional metal chelators. The first function is the metal chelator DTC (sodium diethyldithiocarbamate (Et2DTC)), which has the sole chemical function of specific metal binding. An amphiphile (more precisely, an oligoether) is the second function and has a sole physical function: changing the physical properties of the molecule. The paper discusses novel copper chelators with variable amphiphilic properties: disubstituted dithiocarbamates (DTCs, R-1(CH2CH2O)(2)(NRCS2Na)-C-2) with one alkyl (R-2 = hexyl, octyl, decyl, or dodecyl) and one oligoether (R-1(CH2CH2O)(2), where RI = Et or Bu) substituent. The octanol-water distribution ratios of the dithiocarbamates (partition measurements by the shake-flask method) and their penetration through the liposome bilayer were measured to predict their transport behavior through biological membranes. The comparative copper binding constants and the stepwise transformation of Fe(DTC)(3) to Cu(DTC)(2) were measured and show the selectivity of the ligands for copper over iron. Differential effects of dithiocarbamates on the rates of copper removal from SOD are shown. The influence of DTCs on SOD superoxide dismutation activity was measured by the cytochrome C/xanthine/xanthine oxidase assay. The SOD dismutation activity was recovered after incubation of inactivated SOD with copper. Inhibition of peroxidase activity of SOD by different DTCs was determined using electron paramagnetic resonance spectra of the DMPO-(OH)-O-. adduct formed in solutions containing H2O2 and CuZnSOD and in the presence of the spin trap compound. The conclusions are that the addition of an oligoether chain of up to eight aliphatic carbon atoms to the structure of dithiocarbamates leads to an increase in the hydrophobicity (relative to Et2DTC) of more than 1000-fold but only a 2.3-fold decrease in the ability to inhibit SOD dismutation activity. However, the rates of decomposition of Na, K, and Li salts are tremendously enhanced. These unexpectedly high rates of hydrolysis may be due to high interfacial activity as may be deduced from the preliminary interfacial tension and critical micelle concentration data. DTCs have close similarity in their ability to transport through bilayer membranes when the rates of substitution of CU2+ by DTCs from calcein-copper complex in buffer solution and in liposomes are compared. Structural attenuation leads to a balance in desired properties. Library members with decyl and dodecyl groups are poor inhibitors of SOD dismutase activity. Amphiphilic dithiocarbamates (hexyl and octyl substituted) reduce the peroxidase activity of SOD. Amphiphilic DTCs and their copper complexes do not cause damage to a tested membrane model (liposome system) and, hence, are not expected to damage biological membranes.