Weight and Copper Content

100 from a crystallographic measurement of the unit ceU volume, the density of the crystals, and careful determination of the partial specific volume of the protein 133). Later, Ryden 122) reported a molecular weight of 134,000 

Freeman and Daniel 226) one of 124,000 both being based on sedimentation-equilibrium studies, and in addition, the amino acid and carbohydrate composition suggest a molecular weight near 130,000 119). These studies appear to establish the molecular weight of ceruloplasmin to be near 124,000—134,000. Morell et al. 134) have carefully determined the Cu content and dry weight of the protein finding a value of 0.275 0.009%, w/w Cu which for a molecular weight of 

100 corresponds to 5.73 0.36 Cu per mole ceruloplasmin. This number is reasonably rounded to 6 Cu per mole protein. 

However, the presence of 6 Cu atoms requires that the magnetic properties of Cu bound to ceruloplasmin be rationalized in a presently unacceptable manner. The essence of this dilemma is that while all Cu bound to ceruloplasmin is formally Cu + 55) only 44% of this Cu is paramagnetic 58, 135). This would correspond to 2.6 Cu2+ and 3.4 non-paramagnetic Cu atoms, numbers which deviate too greatly from the integer values required by the magnetic properties. 

We turn now to the physical properties of the Cu bound to ceruloplasmin. The EPR and visible optical spectra suggest considerable similarity between the Cu of ceruloplasmin and that of laccases. Thus, as first reported by Holmberg and Laurell 101) the optical spectrum is characterized by absorption bands at 610 nm and 330 nm. The EPR spectrum shows the presence of both Type 1 and Type 2 Cu + ions and a significant fraction of the Cu atoms are EPR-nondectable, 56% as compared to 50% in laccase 58, 135). Thus, there appear to be only three general types of Cu binding sites in ceruloplasmin, and the number of each type of Cu has recently been determined with considerable certainty. 

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There has been much uncertainty over the molecular weight and copper content. It appears to involve a single polypeptide chain of molecular weight about 130 000 with six or seven copper ions. On balance, there are two type 1, one type 2 and four type 3 copper centres. This is in accord with the fact that 44% of the total copper is paramagnetic and ESR-detectable.977... 

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