Many genetic variants of alpha(1)-antitrypsin (alpha(1)AT) are associated with early onset emphysema and liver cirrhosis. We previously found that although the stability and inhibitory activity of the human alpha(1)AT variant M-malton (Phe52-deleted) are comparable to those of wild-type (alpha1)AT, the M-malton variant spontaneously undergoes a conformational change to a more stable, inactive, latent form under physiological conditions. Here, we show that insertion of an exogenous peptide having a sequence corresponding to the first strand of beta-sheet C (s1C) is facilitated in M-malton alpha(1)AT, suggesting that the endogenous s1C and reactive center loop are easily released from beta-sheet C, thus promoting latency conversion. When additional stabilizing mutations were introduced into Mmalton aIAT, they suppressed the conformational defect of this variant: the latency transition was greatly retarded, presumably by strengthening the interactions between s1C and beta-sheet C. (C) 2004 Elsevier Inc. All rights reserved.