Nuclear-charge yields of fragments produced by fission of 214,…,223Ac, 220,…,229Th, 224,…,232Pa, and 231,…,234U have been measured. These radioactive nuclei were produced as secondary beams, and fission was induced in flight at 420 A MeV by electromagnetic excitation in a lead target. The excitation-energy distribution at fission is estimated to be centered at 11 MeV. The nuclear-charge distributions of the fission fragments of odd- Z actinium and protactinium isotopes show a pronounced odd-even structure: in asymmetric charge splits, the unpaired proton predominantly sticks to the heavy fragment. A model is proposed which attributes this effect to the single-particle level densities of the nascent fission fragments. This model also explains the strongly enhanced odd-even effect observed in the most asymmetric parts of the nuclear-charge distributions after fission of the even- Z nuclei 220,…,229Th and 231,…,234U. It is concluded that proton pairs are broken in an early stage of the fission process.