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SummaryWe describe a new method for determining the biological activity of heparin in plasma with use of thrombin and the substrate Tos-Gly-Pro-Arg-pNA. The procedure is based on the photometric determination of the inactivation of thrombin after incubation with plasma in the presence of endogenous antithrombin III (At III). The method allows the specific determination of heparin concentrations from 0.02 USP to 0.8 USP/ml of plasma in the presence of normal At III levels. It has been carried out manually by use of an Eppendorf spectrum line photometer or automatically by use of a Vitatron Akes analyzer. For evaluation, the results were compared with two standard samples which contained heparin in the low and high therapeutic range, respectively.

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1. Starting from trans-cinnamic acid a chiral ( -)3-phenyl-[2,3-zH]propionic acid has been synthesized using Clostridium kluyveri cells as catalyst. 2.The chiral dideuterated acid has been converted by chemical methods to a mixture of (2R) and (2S)-phenyl[2,3-2H]-alanine.3. By means of 'H nuclear magnetic resonance spectroscopy and the action of D and L-amino-acid oxidase the configuration of the phenylalanine has been shown to be (2R, 3s) and (2S, 3S), respectively. The labelled phenylalanine is thus sterically and isotopically homogeneous at position 3 but heterogeneous at position 2.

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