RESUMO
Phosphoribulokinase (EC 2.7.1.19, ATP: d-ribulose-5-phosphate-1-phosphotransferase) resembles the NADPH-dependent glyceraldehyde-3-phosphate dehydrogenase (EC 1.2.1.13, d-glyceraldehyde-3-phosphate: NADPH(+) oxidoreductase [phosphorylating]) of chloroplasts in that the activation of both of these enzymes involves the dissociation of oligomers (apparently tetrameric forms) with low catalytic activity to give protomers which possess higher catalytic activity. Gel filtration on Sepharose 6B has shown that the molecular weights of the oligomer and active protomer of phosphoribulokinase are, respectively, about 6.8 x 10(5) and 1.7 x 10(5), whereas the corresponding values for glyceraldehyde-3-phosphate dehydrogenase are 8.2 x 10(5) and 2.2 x 10(5). Activation of both enzymes occurs in response to either ATP, dithiothreitol, or cholate while the glyceraldehyde-3-phosphate dehydrogenase is also activated by NADPH. Activation/dissociation of these enzymes may involve conformational changes resulting from nucleotide binding, the reduction of sulfur bridges, and the cholate induced loosening of hydrophobic interactions.
RESUMO
At concentrations of up to 300 mug/ml both d-threo- and l-threo-chloramphenicol act as energy transfer inhibitors in spinach chloroplasts, in that they inhibit both phosphorylation and phosphorylating electron transport, without affecting the nonphosphorylating electron transport which occurs either in the absence of a phosphate acceptor or in the presence of the uncoupler ammonium chloride. At higher concentrations, there appears to be an additional site of chloramphenicol inhibition of electron transport. If d-threo-chloramphenicol is to be used as a protein synthesis inhibitor in intact chloroplasts or tissues, control experiments with another chloramphenicol isomer seem to be necessary.
