alpha 1-Antitrypsin microheterogeneity. Isolation and physiological significance of isoforms

alpha 1-Antitrypsin has a microheterogeneity evident on isoelectric focusing as three major and several minor bands. We have identified the carbohydrate structures of the major bands; band 6 (isoform I) has three bi-antennary sidechains, band 4 (isoform II) has two bi- and one tri-antennary and band 2 (isoform III) has one bi- and two tri-antennary sidechains. The identity of the isoforms with the bands permitted their measurement in plasma by photometric scanning of the electrofocused gels. In healthy controls the levels of isoforms I, II and III were relatively constant and in the proportions of 5, 4 and 1, respectively. A marked change occurred during inflammation and oestrogen stress with isoforms II and III accounting for most of the increase in alpha 1-antitrypsin. One possible consequence of the changed proportions was shown to be the increased catabolism of the partially desialylated tri-antennary isoforms compared to that of the predominant bi-antennary form of the healthy individual.