1. A novel use of a double-isotope method is described which allows radioenzymatic assays to combine precision and sensitivity.

2. In the catechol O-methyltransferase assay separate portions of each plasma sample are incubated with either S-[3H]- or S-[14C]-adenosyl-l-methionine. Standards of noradrenaline and adrenaline are added to the latter portions and are thus converted into standards of [14C]metadrenalines. These are added to the 3H-labelled portions after the incubation, where they function as tracers.

3. The final recovery of 14C radioactivity corrects for (a) the efficiency of methylation in the plasma sample concerned and (b) the recovery of metadrenalines during the extraction procedures.

4. The 3H/14C ratio is constant in each assay for a given catecholamine concentration and is determined for samples to which standards of noradrenaline and adrenaline are added to the 3H- (as well as the I4C-) labelled portions before the initial incubation.

5. The sensitivity of the assay is increased by using high specific radioactivity S-[3H]adenosyl-l-methionine (60-85 Ci/mmol), and low backgrounds are maintained by catecholamine depletion in vivo in the rats used for enzyme preparation.

6. Both catecholamines (1.5 pg/ml; 10 pmol/l) may be detected; the coefficients of variation are 3.0 and 3.2% for noradrenaline and adrenaline respectively (intra-assay) and 4.6 and 5.0% (inter-assay).

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