The holoenzyme is denatured by heat at very high temperature.  It is found that two polypeptides, although denatured, contains chains that remain together as a unit, while two other denatured polypeptides are floating free. You interpret this to mean that:

I.    The primary structure of the subunits changed during the high temperature heat treatment.

II.   A disulfide bridge was present linking only two of the subunits in holoenzyme.

III.  Ionic bonds, hydrogen bonds, and hydrophobic interactions were disrupted during the heat treatment.