A single polypeptide acts both as the beta subunit of prolyl 4-hydroxylase and as a protein disulfide-isomerase.

A single polypeptide is shown to act both as the beta subunit of the proline hydroxylase (EC 1.14.11.2) and as a protein disulfide-isomerase (EC 5.3.4.1). When isolated from chick embryos or rat liver, the beta subunit of prolyl 4-hydroxylase and the enzyme protein disulfide-isomerase have identical molecular weights and peptide maps as produced by digestion with Staphylococcus aureus V8 protease. The apparent molecular weights of both proteins isolated from human placental tissue are slightly higher, and the human beta subunit and one of its peptides have molecular weights about Mr 500 higher than the protein disulfide-isomerase and its corresponding peptide. Experiments with polyclonal and monoclonal antibodies also suggest a structural identity between the two proteins. The beta subunit isolated from the prolyl 4-hydroxylase tetramer has protein disulfide-isomerase activity similar to protein disulfide-isomerase itself, and even the beta subunit when present in the prolyl 4-hydroxylase tetramer has one-half of this activity.