Takeshi Kagawa

Regulation of C4 photosynthesis: Characterization of a protein factor mediating the activation and inactivation of NADP-malate dehydrogenase

Abstract The leaf NADP-malate dehydrogenase of Zea mays is rapidly activated when leaves are illuminated and inactivated in the dark. The present studies show that inactive enzyme isolated from darkened leaves was activated by dithiothreitol and that the active enzyme was rapidly inactivated by oxygen in dithiothreitol-free solutions. Following the fractionation of leaf extracts, both the activation and inactivation of NADP-malate dehydrogenase in vitro were partially or totally dependent upon a separate small molecular weight protein factor. Activation and inactivation were largely or solely dependent upon this factor at pH 8.0 or less, but apparently only partially factor dependent at pH 9.0. The factor was heat stable, inactivated by incubation with trypsin, and had a molecular weight of about 10,000. It was mostly associated with the chloroplasts of mesophyll cells.

On the light dependency of fatty acid synthesis by isolated spinach chloroplasts

Abstract When intact isolated spinach chloroplasts were incubated with [ 14 C]-acetate, CoA and ATP, acetyl CoA accumulated in the medium, both in light and darkness. The accumulation of this acetyl CoA (> 400 nmol mg chlorophyll −1 h −1 ) was apparently brought about by a thiokinase in an outer chloroplast compartment and was not due to a dark-block of acetyl-CoA carboxylase since high rates of acetate incorporation into fatty acids (> 1000 nmol mg chlorophyll −1 h −1 ) could be induced whether or not acetyl CoA accumulated. Maximum dark rates of fatty acid synthesis in gently broken chloroplasts were only 2% of those achieved by intact chloroplasts in the light.

Lecithin synthesis during microbody biogenesis in watermelon cotyledons.

Abstract During the normal development of watermelon seedlings, leaf peroxisomes succeed glyoxysomes as the major microbody component in the cotyledons. The possibility has thus been raised that the two organelles are ontogenetically related; that leaf peroxisomes are derived from glyoxysomes. The behavior of lecithin, an important constituent of the membranes of both kinds of organelle was examined in this study. Using labeled choline as a precursor of lecithin, its incorporation into various membrane fractions was followed during the period when glyoxysomal activity was declining and that of leaf peroxisomes increasing after exposure to light. The results showed that glyoxysomal membrane was selectively destroyed during this period. Furthermore, from double-labeling experiments using [ 14 C]- and [ 3 H]choline it was shown that newly synthesized lecithin was incorporated into the membranes of the developing leaf peroxisomes. These results support the thesis that leaf peroxisomes are not derived from glyoxysomes and instead represent two distinct microbody populations.