A determination of the sub-units of arachin by osmometry. Arachins A, B, and A1.

1. Osmotic pressure determinations of dissociated arachins are a particularly suitable method for determination of the number of sub-units in the protein, because they yield a number-average molecular weight. 2. Arachin, in 8m-urea-0.1m-sulphite, produces 12 sub-units from the form of molecular weight 345000. 3. When the urea concentration is varied the molecules became fully dissociated at 6m-urea-0.1m-sulphite. Although sulphite is necessary to break disulphide bridges, concentrations greater than 0.1m cause a re-aggregation of the sub-units. Similar results were obtained in guanidine solutions. 4. A new form of arachin has been discovered, A1, migrating more rapidly than arachin A. 5. The N-terminal residues of arachin have been re-investigated on more highly purified samples: they are glycine, valine and (iso)leucine in the proportions 4:1:1. 6. The three forms of arachin have the structure (B) beta(4)gammadelta, (A) alpha(2)beta(2)gammadelta and (A1) alpha(4)gammadelta, for the forms of molecular weight 170000. 7. Dissociation in 8m-urea produces some fragments, detected by gel electrophoresis, which appear to be dimers of the type alpha-S-S-beta, beta-S-S-beta, held together by disulphide bonds.