8-epi-PGF2, which has been proposed as a marker of oxidative
stress, was measured by using an enzyme immunoassay kit
(37). First, samples (0.5–1 mL) were mixed gently with 50 L
affinity sorbent (mouse anti-8-isoprostane covalently bound to
Sepharose 4B) (Cayman Chemical Co, Ann Arbor, MI) for 60 min
at room temperature to purify the samples. Next, the samples were
briefly centrifuged at 11 750 g (4 C) so that the sorbent would
form a sediment (Cayman Chemical Co). The sorbent, which contains
the bound 8-isoprostane, was rinsed with washing solution,
and, after the supernatant fluid was discarded, the sorbent pellet
was resuspended in 300 L of an ethanol elution solution. The
ethanol washes were evaporated to dryness in a vacuum centrifuge
and were immediately dissolved in 125 L enzyme
immunoassay buffer. Then, samples were analyzed in duplicate
by using the enzyme immunoassay kit. This assay is based on the
competition between 8-isoprostane and an 8-isoprostane-acetylcholinesterase
conjugate (8-isoprostane tracer) for a limited
number of 8-isoprostane-specific rabbit antiserum binding sites.
The rabbit antiserum 8-isoprostane (either free or tracer) complex
binds to the rabbit immunoglobulin G mouse monoclonal antibody
that was previously attached to the well. The plate was washed to
remove any unbound reagents and then Ellman’s reagent (which
contains the substrate to 8-isoprostane-acetylcholinesterase; Cayman
Chemical Co) was added to the well. The product of this enzymatic
reaction has a distinct yellow color and absorbs strongly at
412 nm. The intensity of this color, determined spectrophotometrically,
is proportional to the amount of 8-isoprostane tracer
bound to the well, which is inversely proportional to the amount
of free 8-isoprostane present in the well during the incubation. The
concentration of 8-isoprostane in the test samples was interpolated
from the standard curve by using log transformation (16).