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 o f the most important enzymes of the antioxidant defense system of aerobic o rganisms, is the subject of this paper. It is desirable in several noted me dical and agricultural applications. This paper describes the synthesis of a very limited "sublibrary" of bifunctional metal chelators. The first func tion is the metal chelator DTC (sodium diethyldithiocarbamate (Et2DTC)), wh ich has the sole chemical function of specific metal binding. An amphiphile (more precisely, an oligoether) is the second function and has a sole phys ical function: changing the physical properties of the molecule. The paper discusses novel copper chelators with variable amphiphilic properties: disu bstituted dithiocarbamates (DTCs, R-1(CH2CH2O)(2)(NRCS2Na)-C-2) with one al kyl (R-2 = hexyl, octyl, decyl, or dodecyl) and one oligoether (R-1(CH2CH2O )(2), where RI = Et or Bu) substituent. The octanol-water distribution rati os of the dithiocarbamates (partition measurements by the shake-flask metho d) and their penetration through the liposome bilayer were measured to pred ict their transport behavior through biological membranes. The comparative copper binding constants and the stepwise transformation of Fe(DTC)(3) to C u(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 dismuta tion activity was measured by the cytochrome C/xanthine/xanthine oxidase as say. The SOD dismutation activity was recovered after incubation of inactiv ated SOD with copper. Inhibition of peroxidase activity of SOD by different DTCs was determined using electron paramagnetic resonance spectra of the D MPO-(OH)-O-. adduct formed in solutions containing H2O2 and CuZnSOD and in the presence of the spin trap compound. The conclusions are that the additi on of an oligoether chain of up to eight aliphatic carbon atoms to the stru cture of dithiocarbamates leads to an increase in the hydrophobicity (relat ive to Et2DTC) of more than 1000-fold but only a 2.3-fold decrease in the a bility to inhibit SOD dismutation activity. However, the rates of decomposi tion of Na, K, and Li salts are tremendously enhanced. These unexpectedly h igh rates of hydrolysis may be due to high interfacial activity as may be d educed from the preliminary interfacial tension and critical micelle concen tration data. DTCs have close similarity in their ability to transport thro ugh bilayer membranes when the rates of substitution of CU2+ by DTCs from c alcein-copper complex in buffer solution and in liposomes are compared. Str uctural attenuation leads to a balance in desired properties. Library membe rs with decyl and dodecyl groups are poor inhibitors of SOD dismutase activ ity. 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, a re not expected to damage biological membranes.