Radioenzymatic estimation of noradrenalin in small plasma samples without prior extraction

We have developed a radioenzymatic method for the estimation of noradrenalin in small plasma samples by utilizing partly purified phenylethanolamine-N-methyltransferase. Noradrenalin can be estimated in deproteinised plasma without prior extraction. The sensitivity of the assay is better than 2.5 pg. Inclusion of an internal standard with each sample is necessary as the enzyme reaction is slightly influenced by some components of the plasma. Prior extraction with alumina severely decreases the sensitivity of the assay.