The effects of oxidative stress on the degradation of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco; EC 4.1.1.39) were studied in isolated chloroplasts from barley (Hordeum vulgare L. cv Angora). Active oxygen (AO) was generated by varying the light intensity, the oxygen concentration, or the addition of herbicides or ADP-FeCI,-ascorbate to the medium. Oxidative treatments stimulated association of Rubisco with the insoluble fraction of chloroplasts and partia1 proteolysis of the large subunit (LSU). The most prominent degradation product of the LSU of Rubisco showed an apparent molecular mass of 36 kD. The data suggest that an increase in the amount of AO photogenerated by O, reduction at photosystem I triggers Rubisco degradation. A possible relationship between AO-mediated denaturation of Rubisco and proteolysis of the LSU is discussed.

Six unrelated families with genetically determined structural variants of apo A-I were found in the course of an electrophoretic screening program for apo A-I variants in dried blood samples of newborns. The following structural variations were identified by the combined use of HPLC, time-of-flight secondary ion mass spectrometry (TOF-SIMS), and automated gas phase sequencing: Pro3 --Arg (1X), Pro4 Arg (1X), and Pro165--Arg (4X). All variant carriers were heterozygous for their mutant of apo A-I.Subjects heterozygous for apo A-I(Pro165--Arg) (n = 12) were found to exhibit lower mean values for apo A-I (109±16 mg/dl) and HDL cholesterol (37±9 mg/dl) than unaffected family members (n = 9): 176±41 and 64±18 mg/dl, respectively (P < 0.001). In 9 of 12 apo A-I(Pro165 --Arg) variant carriers the concentrations of apo A-I were below the fifth percentile of sex-matched controls. By two-dimensional immunoelectrophoresis as well as by densitometry the relative concentration of the variant apo A-I in heterozygous carriers of apo A-I(Pro165 --Arg) was determined to account for only 30% of the total plasma apo A-I mass instead of the expected 50%. Thus, the observed apo A-I deficiency may be largely a consequence of the decreased concentration of the variant apo A-I.In the case of the apo A-I(Pro3-) Arg) mutant, densitometry of HDL apolipoproteins demonstrated a distinctly increased concentration of the variant proapo A-I relative to normal proapo A-I. This phenomenon was not observed in the apo A-I(Pro4 --Arg) mutant or in other mutants. This suggests that the interspecies conserved proline residue in position 3 of mature apo A-I is functionally important for the regular enzymatic conversion of proapo A-I to mature apo A-I.

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