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Summary. A simple and reliable method for the estimation of cytidine deaminase (cytidine aminohydrolase; EC 3.5.4.5) activity in pregnancy serum is described. This enzyme does not require magnesium for activation and is also more stable than deoxycytidylate deaminase (dCMP deaminase, dCMP aminohydrolase; EC 3.5.4.12). There was excellent correlation between the two enzymes (r= 0.92). Both enzymes showed increased activity in abnormal pregnancy. Both enzyme activities were found to be similar in 1305 maternal serum samples and therefore due to the simplicity of cytidine deaminase estimation it is recommended for the screening of large numbers of antenatal sera for abnormal pregnancies.

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Adenosine deaminase (AD), a purine salvage enzyme, exists as AD isoform 1 (AD1) and AD isoform 2 (AD2). Plasma AD has been advocated for the screening and monitoring of cancer, as AD2 activity is increased in conditions associated with tumour growth. Plasma AD2 was measured before and seven to 10 days after the first dose of chemotherapy in patients with different tumours. A 'tumour regression score' was assessed independently based on radiological changes seen in the tumour following completion of chemotherapy. Changes in plasma AD2 were then compared with the tumour regression score. Following first-dose chemotherapy, plasma AD2 decreased on average from 22.7 +/- 10.5 U/L to 15.0 +/- 4.6 U/L. The percentage decrease in plasma AD2 correlated with the tumour regression score (r=0.5, P=0.028). These data suggest plasma AD2 may have a role in determining tumour response to treatment.

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