Following exposure, neonicotinoid insecticides (NEOs)
can be metabolized
by both Phase I and Phase II reactions catalyzed by human cytochrome
P450 enzymes. However, toxicities of parent NEOs and their metabolites
are still unclear, and little is known about biotransformation rates
and pathways of NEOs in humans. In this study, 98 serum samples collected
in China were analyzed for free, conjugated and total forms of six
parent NEOs (i.e., acetamiprid (ACE), imidacloprid (IMI), clothianidin
(CLO), thiacloprid (THD), thiamethoxam (THM), and dinotefuran (DIN))
and four metabolites (i.e., N-desmethyl-acetamiprid
(N-dm-ACE), 1-methyl-3-(tetrahydro-3-furylmethyl)
(DIN-U), 5-hydroxy-imidacloprid (5-OH-IMI), olefin-imidacloprid (Of-IMI)).
NEOs and their metabolites were detected in all serum samples, and
the total median concentrations of free, conjugated, and total forms
of 10 NEOs were 2.04, 2.01, and 5.12 ng/mL, respectively. Conjugated
forms of NEOs accounted for only half (53%) of the total forms of
NEOs. Based on the profiles of Phase I and Phase II metabolites of
NEOs in serum, it was found that age is a determinant in Phase I metabolism
of DIN and Phase II metabolism of IMI. The Phase II metabolites of
NEOs are associated with oxidative DNA damage, and the conjugated
forms of IMI, DIN, and 5-OH-IMI in serum were significantly positively
correlated with oxidative stress. Overall, the amount of NEOs present
in conjugated forms in human serum was determined to document the
existence of a considerable proportion of free forms of these insecticides.