. . . "wherein, when Z is an aldehyde group or a keto group, A comprises an amino group forming the linkage with Z, and the protein is selected from the group consisting of IFN beta, GM-CSF, APC, tPA, A1AT, AT III, factor VII, factor VIII, and factor IX, wherein, when Z is an amino group, A is selected from the group consisting of a reactive carboxy group and an aldehyde group, a keto group or a hemiacet" .