An isolated protein having at least 90% homology with the dimeric protein having the following amino acid sequence (I) with the proviso that at the C-terminal end of the monomeric forms, 2 to 10 amino acid distance from Z.sup.2, a Cys is present for forming the dimeric or multimeric protein, Z.sup.1 is a signal peptide for protein transport to the ER and/or through the plasma membrane or NH.sub.2 or a derivative of an NH.sub.2 group such as an alkylated, or acylated derivative, or polyethylene glycol (PEG) moiety, Z.sup.2 is an amino acid residue with up to 20, in particular 10 or 8 amino acids which are selected of substantially non-hydrophobic naturally occurring amino acids or COOZ.sup.3 wherein Z.sup.3 is hydrogen, a metal ion, a hydrocarbon moiety substituted or non-substituted with heteroatoms.