REEVALUATION OF THE COLORIMETRIC ASSAY FOR CYTIDINE DEAMINASE ACTIVITY

This study re-evaluated the colorimetric assay for cytidine deaminase (CTD), and showed that the optimum conditions were pH 7.5, 37-degrees- C, and up to 24 h. In addition, this method was found to require prote in precipitation. Following these modifications, intra-assay and inter -assay coefficients of variation were below 5 per cent, indicating tha t the assay was highly reliable. CTD activity was determined in 282 se rum samples from 206 normal pregnant women by the incubation of 100 mu l of serum and 400 mul of 1.4 mmol/l cytidine substrate for 16 h at 37 -degrees-C. Following protein precipitation, the ammonia liberated dur ing conversion was measured by a colorimetric procedure. The mean (+/- SD) CTD activity was 7.31 +/- 2.50 U at 3-12 weeks of gestation, 8.70 +/- 2.12 U at 13-24 weeks, 7.59 +/- 2.25 U at 25-36 weeks, and 7.29 /- 2.16 U at 37-42 weeks. High levels of CTD activity were found in pa tients with abruptio placentae and amnionitis associated with intraute rine fetal death (IUFD). The increase in CTD activity was noted from 3 days to 1 week before the confirmation of IUFD. The placenta contains extremely high levels of CTD, but cord serum does not. Thus, the exce ssive elevation of CTD activity was probably derived from progressive placental damage. This modified CTD assay was concluded to be simple a nd reliable, and may perhaps be useful in detecting pregnancy disorder s.