Summary The protease inhibitory spectra of the eight homozygous Thoroughbred Pi types against trypsin, elastase and chymotrypsin have been determined. The α1‐protease inhibitor proteins exhibit three classes of inhibitory specificity towards these enzymes. The Pi types F, I, N and U exhibit class I (trypsin, elastase and chymotrypsin) and class II (trypsin and elastase) types of inhibition and fit Juneja et al.s (1979) classification of two separate genetic systems Pi 1 and Pi 2 based on differences in the inhibitory spectra against trypsin and chymotrypsin. The remaining four Pi types are exceptions to Juneja et al.s (1979) classification. Types G, L, S 1 and S 2 possess class I but not class II proteins. A third class of proteins (class III) which exclusively inhibit chymotrypsin was detected in all eight protease inhibitor types. Type G is well represented by class III proteins because two of the three major proteins of the ISO‐DALT pattern inhibit only chymotrypsin and is thus an exception to Juneja et al.s (1979) classification.
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Pollitt et al. (1983) studied this question.