In the general frame of a project aiming at screening candidate drugs against several different matrix metalloproteinases (MMP) to find a rationale for selectivity, the catalytic domain of MMP-12 (metalloelastase) has been expressed in E. coli strain BL21D3 and its production optimized to about 30 mg/dm(3). The chosen construct spans residues 106-267 of the whole MMP-12 and contains two additional methionines at positions 104-105. This is at variance with the previously published constructs which span residues 99-279 [J. Mol. Biol. 312 (2001) 743] and 100-262 [J. Mol. Biol. 312 (2001) 7311, respectively. The protein, expressed in inclusion bodies, is solubilized in high urea concentration and properly refolded, as judged from 1 H to N-15 HSQC, by stepwise urea dilution in the presence of the mild inhibitor acetohydroxamic acid. The latter can be easily dialysed out when needed for activity or inhibition studies. The solubility of this catalytic domain construct of MMP-12 is around 250-300 muM. To increase solubility, a mutant (F171D) has been designed that should not alter the activity and should not interfere with the contacts between the catalytic domain and either the pro-domain or the C-terminal domains that precede and follow it in the full-length protein. The F171D mutant was produced and indeed resulted fully active and three times more soluble than the WT, greatly facilitating its use in NMR screening experiments. Comparison of X-ray data for the present [Angew. Chem. Int. Ed. 42 (2003) 2673] and previous [J. Mol. Biol. 312 (2001) 73 1; J. Mol. Biol. 312 (2001) 743] constructs of the catalytic domain of MMP-12 suggested that elimination of Met 105 and of the stretch 264-267 should lead to an even more soluble and stable construct. The latter was produced and showed to have the desired properties. (C) 2003 Elsevier B.V. All rights reserved.