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HAEM-PROTEIN REDUCTION BY COLOROPLASTS 501 to the conclusion that the factor can catalyse the reduction of soluble material in the leaf extracts. Further work is necessary to trace the paths of hydrogen transport after the reduction of the factor by the illuminated chloroplast and to determine the nature of the group concerned in the oxidation-reduction cycle. SUMMARY 1. A protein factor from leaves, previously shown to be active in catalysing the reduction of methaemoglobin and metmyoglobin by illuminated chloroplasts, has been purified by fractionation with ammonium sulphate followed by electrophoretic separation of the active material on paper. 2. The purified protein was found to give a single symmetrical boundary in the ultracentrifuge and in the Tiselius electrophoresis apparatus. 3. The molecular weight of the protein, calculated from sedimentation and diffusion measurements, is 19 000. 4. The purified protein is highly active in catalysing the photochemical reduction of metmyoglobin and also stimulates the reduction of cytochrome c, cytochrome b3 and the cytochrome components present in a particulate heart-muscle preparation. 5. The photochemical reduction is inhibited by organic mercury compounds and this inhibition is partially reversed by cysteine. 6. The protein does not catalyse the reduction of haem-proteins when coenzymes served as hydrogen donor. Illuminated chloroplasts were the only effective hydrogen-donating system observed. 7. The chemical nature of the group conferring oxidation-reduction properties to the protein has not yet been determined but neither flavin nor haem could be detected. 8. Metmyoglobin-reducing activity has been observed in extracts of chloroplasts and was shown to represent one-third of the activity extractable from the whole leaf. In addition to the acknowledgements made in the text the authors wish to acknowledge the help of Mr F. W. Arnold and the late Mr H. Pentelow in the collection of leaf material, and to thank Mr B. E. Boon for his assistance with the ultracentrifuge runs and Mr B. R. Slater for the diffusion measurements. This work formed part of a programme supported by the Agricultural Research Council.

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The heavy chain of a pathological human immunoglobulin IgG and also the Fd fragment have been isolated. No free alpha-amino group was present on either and the N-terminal sequence of both has been identified as pyrrolid-2-one-5-carbonylvalylthreonine. Splitting at the four methionine residues of the heavy chain with cyanogen bromide gave five fractions. The fraction from the C-terminal end of the chain was isolated in high yield and the amino acid sequence was: His-Glu-Ala-Leu-His-Asp(NH(2))-His-Tyr-Thr-Glu(NH(2))-Lys-Ser-Leu-Ser-Leu-Ser-Pro-Gly These results give strong support to the view that the heavy chain of immunoglobulin is a single peptide chain.