Page 29 - Volume 6, Issue 2, July 2013

Biodegradation of diazinon by epiphytic yeasts

77

be seen in Figure 5, the interaction of diaz-

inon in the activity of glutathione-S-trans-

ferases (GSTs) of both yeasts was obvious.

Diazinon inhibited GSTs activity reaching

the maximum inhibition of 100% at concen-

tration of 79 μΜ. The IC

50

(half maximal in-

hibition concentration- IC

50

) values calcu-

lated were 39 μΜ for

Rh. glutinis

and 52 μΜ

for

Rh. rubra

. On the other hand, diazinon

did not reduce the activity of esterases at

the concentrations tested and under assay

conditions described (Figure 5). These re-

sults provided an indication that GSTs and

not esterases mediate the observed biodeg-

radation of diazinon by

Rh. glutinis

and

Rh.

rubra.

Although most studies support the

esterase-hydrolase-based biodegradation

of diazinon in hydrolases (Munnecke, 1976;

Barik and Munnecke, 1982 and Sethunathan,

1989), GSTs have been previously shown to

be capable of degrading organophosphates

(Klonis, 2007).

Effect of synergists in biodegradation

rates

The possible effect of enzymatic inhibi-

tors in the biodegradation of diazinon was

examined in 250 mL flasks with mineral

salts medium without glucose since in this

medium both yeasts started the consump-

tion of the insecticide from the beginning

of the trials. After the incubation of the mi-

croorganisms with 1, 5, 10 and 15 μg/mL of

the synergists triphenyl phosphate, dieth-

Figure 3.

Production of the metabolite 2-isopropyl-6-methyl-4-pyrimidinol (IMP) after the biodegradation of 10 and 25

μg/mL diazinon by the yeasts

Rhodotorula glutinis

and

Rhodotorula rubra

in mineral salts nutrient mediumwith and without

glucose. Each value is the mean of three replicates with error bars representing the standard deviation of the mean.