NEW DATA ON THE PROTEINS OF RABBIT (ORYCTOLAGUS-CUNICULUS) MILK

The main rabbit milk proteins have previously been prepared by reversed-phase HPLC of the acid-precipitated material ('whole casein') and of its supernatant (acid whey). Most of them were nearly homogeneous on SDS-PAGE, Among those isolated from whole casein, alpha s1-, beta- and kappa-caseins, as well as whey acidic protein (WAP) were identified by N-terminal sequencing, After further internal sequencing, two unknown proteins were found to be the putative products, alpha(s2a)-and alpha(s2b)-caseins of two recently sequenced transcripts from rabbit mammary gland. Each whole casein component gave several bands on IEF. For kappa-casein, this was probably due to uneven glycosylation as in all kappa-caseins studied so far. For the other whole casein components, including WAP, the number of bands roughly reflected the number of potential phosphorylation sites predicted from the sequences. For alpha(s1)- and alpha(s2)-caseins polymorphism could be detected. From acid whey, in addition to WAP, which was a minor component, reversed phase HPLC separated three proteins, These were alpha-lactalbumin, transferrin and serum albumin, on the basis of their apparent molecular weights deduced from SDS-PAGE. WAP was a major component of the native whey obtained by ultracentrifugation of rabbit milk. It was found to consist of two identical subunits linked by at least one disulfide bridge.