Association of Zn, Cu, Cd and Pb with protein fractions and sub-cellular partitioning in the digestive gland of Octopus vulgaris living in habitats with different metal levels

Zinc, Cu, Cd and Pb concentrations were determined in protein fractions of digestive gland and in the whole digestive gland of Octopus vulgaris collected from two areas of the Portuguese coast. Approximately 95% of Zn, 99% of Cu, 85–96% of Cd and 77–86% of Pb were stored in the cytosol, suggesting the predominance of cytosolic proteins in the trapping these elements. Gel filtration chromatography evidenced the presence of two major groups of proteins, with high molecular weight (HMW, 144 000–130 000 Da) and low molecular weight (LMW, 11 000–6000 Da). The following metal–protein associations were found: Zn was distributed between HMW and LMW; Cu and Cd in LMW proteins with a minor association with HMW; and Pb in HMW proteins. The strong positive correlations between Cd, Zn and Cu and LMW proteins point to the presence of metalloproteins with high affinity to these elements. A shift was registered between the maximum of the ratio 254:280 nm and metal concentrations in the chromatographic profiles. This shift may result from metallothioneins having a small participation in the metal binding or protein purification was insufficient and various LMW proteins may be interfering.