Re-evaluation 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°C, and up to 24 h. In addition, this method was found to require protein precipitation. Following these modifications, intra-assay and inter-assay coefficients of variation were below 5 per cent, indicating that the assay was highly reliable. CTD activity was determined in 282 serum samples from 206 normal pregnant women by the incubation of 100 μl of serum and 400 μl of 1·4 mmol/l cytidine substrate for 16 h at 37°C. Following protein precipitation, the ammonia liberated during 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 patients with abruptio placentae and amnionitis associated with intrauterine 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 excessive elevation of CTD activity was probably derived from progressive placental damage. This modified CTD assay was concluded to be simple and reliable, and may perhaps be useful in detecting pregnancy disorders.