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Hemocyanins (Hcs) are respiratory glycoproteins occurring freely dissolved in the hemolymph of many arthropodan and molluscan invertebrates. They belong to the family of type-3 copper proteins as they contain active sites consisting of paired copper atoms (CuA, CuB). Molluscan Hc subunits are long polypeptide chains containing 7 or 8 so-called functional units (FUs) named a to g or h , from the N-terminus on. Each FU (molecular mass of 50 kDa) contains one dicopper active site and is able to bind a dioxygen molecule. Ten subunits form a decameric entity with a cylindrical shape (molecular mass of 350-400 kDa) in the cephalopodan Hc while a didecameric form (molecular mass of 900 kDa) is characteristic for gastropodan Hc. In this thesis we have investigated the intrinsic and induced phenoloxidase (PO) activity of the Hcs of the gastropod Helix pomatia ( Hp ) and the cephalopod Sepia officinalis ( So ). The small intrinsic o -diPO activity of Hp beta-Hc observed with catechol as substrate is increased by about 3 fold on dissociation of the Hc into dimers of subunits. Further amplification of the PO activity was obtained on treatment with subtilisin, producing internal cleavages inside FUs. The enzymatic efficiency was found to be very low ( e.g. , k cat/ K m= 0.068 mM-1min-1 for associated Hp beta-Hc) compared to a true tyrosinase (Ty). This low activity could be compensated for by the high concentration of Hc in the hemolymph. The K m value of the dissociated Hp beta-Hc and So Hc (~ 3-4 mM) is comparable to that of catechol oxidase and Ty indicating that catechol can complex with high affinity with at least some of the active sites of native Hc molecules. Study of separated FUs showed that the intrinsic PO activity of Hp beta-Hc is basically contributed by FU  Hp f while in So Hc the main contribution comes from FU So g . Also, under the experimental conditions only those two FUs were found to be inducible for enhanced PO activity by limited proteolysis with subtilisin. These data support the earlier conclusion that Hp f and So g are functional and structural analogues. With minute amounts of sodium dodecyl sulphate (SDS) a transient induction of PO activity was observed for Hp beta-Hc. It was observed that at higher SDS concentration ( e.g. , 4 mM) PO activity was lost completely after some time while a small amount of residual copper was retained by the Hc. We also determined the immunological relationship between Hp beta-Hc and Hp alpha-macroglobulin (alpha-M), a high molecular mass proteinase inhibitor which together with Hc is present in the hemolymph. Only the carbohydrate containing Hp FUs contributed in the cross-reaction indicating a possible involvement of sugar in the antigenicity of Hc. Such a role of sugar was confirmed by comparative ELISA (enzyme linked immuno-sorbent assay) experiments with Hp g and deglycosylated Hp g using anti- Hp g IgGs and by inhibition ELISA experiments with glycopeptides of Hp g . Results indicated that some of the IgGs (anti- Hp g ) were directed against the glycan parts. From FU Hp e three glycopeptides were isolated. Their glycan chains were found to be partly responsible for the immunological cross-reaction with alpha-M. With a FU of another gastropodan Hc, namely FU Rt H2- e from Rapana thomasiana Hc, a novel  N -linked glycan structure was discovered by mass spectrometric analysis of (chymo)tryptic glycopeptides. Inhibition ELISA experiments indicated that also this glycan structure was involved in the antigenicity of the Hc. Next to the well known hemoglobin, another oxygen carrier protein hemocyanin (Hc) occurs in the animal kingdom. Hcs are high molecular mass glycoproteins found in the molluscan and arthropodan phyla. The cylindrical shaped molluscan Hc is composed of 10 or 20 subunits, each containing 7-8 functional units (FUs). Each FU contains a pair of Cu atoms in its ‘active site’ which can reversibly bind molecular oxygen. A group of enzymes called phenoloxidases (POs), from which Hcs probably evolved, possess a similar active site with paired Cu atoms but in contrast to Hc they oxidize phenolic substrates. Reports indicate that due to the active site similarity Hcs can be induced to carry out PO activity. This phenomenon has been investigated in this thesis for the Hcs of two animals of the molluscan phylum, the gastropod Helix pomatia ( Hp , vineyard snail) and the cephalopod Sepia officinalis ( So , cuttle-fish). Small intrinsic o -diPO activity was noted for the native Hp beta-Hc which was increased by 3 times by dissociating the Hc into dimers of subunits. A limited subtilisin treatment introducing internal cleavages inside the FUs could further increase the PO activity of the dissociated Hp beta-Hc. Also Hp alpha-Hc (with two subtypes, alpha-N-Hc and alpha-D-Hc) and the So Hc exhibited a similar induction in PO activity on subtilisin treatment. Among the 8 FUs, basically only one was responsible for the intrinsic PO activity, namely FU Hp f for Hp beta-Hc and FU So g for So Hc (subunit 2). Also only these FUs were inducible by limited proteolysis with subtilisin. Hp beta-Hc was also induced for the PO activity by treatment with small amounts of sodium dodecyl sulphate (SDS, a detergent). Hcs are strongly immunogenic, i.e. , they can elicit the production of antibodies when injected into an animal. In this thesis we have investigated the involvement of the glycans (sugars attached to the FUs) in the antigenicity of Hc of Hp beta-Hc and the Hc of the marine gastropod Rapana thomasiana ( Rt ). By means of immunological experiments we have gathered evidence that glycans of FUs Hp e and Hp g contribute in the antigenicity of Hp beta-Hc. Likewise, the glycans of Rt H2- e ( Rt FU e , subunit 2), whose structures were also determined in this work, contribute in the antigenicity of Rt Hc. We have isolated and characterized Hp alpha-macroglobulin (alpha-M), a proteinase inhibitor which coexists with Hc in the Hp hemolymph. An immunological relationship between alpha-M and beta-Hc was established. Only the sugar containing Hp FUs were found to be involved in the cross-reaction. It means that these two proteins possess some common antigenic sites most of which are sugar motifs and the antibodies raised against one protein can be detected by the other.