Two sandwich enzyme-linked immunosorbent assays (ELISA) have been
developed for quantitation of bovine milk plasminogen and plasmin. The
assays used
two monoclonal antibodies, one specific for plasminogen and the other specific
for
plasminogen plus plasmin. Plasmin concentration was obtained by subtracting
the
first concentration from the second. The assays were sensitive (linear
range,
5–75 ng/ml), repeatable (CV, 8 and 5% for plasmin
and plasminogen titration
respectively), specific (no cross reactivity with the major milk
proteins) and directly
applicable to skimmed milk with no particular pretreatment of the sample.
However,
ELISA did not permit complete titration of milk plasminogen and plasmin
(only
60–90% could be quantified). This lack of accuracy was due to
casein interfering with
the plasmin–plasminogen titration by ELISA. Results obtained with
ELISA or an
enzymic technique on 20 milk samples collected from individual cows milked
throughout pregnancy showed that the ELISA was particularly suitable for
analysis
of late lactation or mastitic milk where proteinase inhibitors
interfered with enzymic quantitation.