Isoforms of NADP-dependent Malic Enzyme in Tissues of the Greening Maize Leaf

Scagliarini, S., Pupillo, P. and Valenti, V. 1988. Isoforms of NADP-dependent malic enzyme in tissues of the greening maize leaf.—J. exp. Bot. 39: 1109–1119. The compartmentation of the isoforms of NADP-dependent malic enzyme (E.C. 1.1.1.40) has been studied in cell-free extracts and in enzymatically-isolated protoplasts of mesophyll tissue (MT) and bundle sheath (BS) strands of greening maize leaves. The etiolated leaf of 10-d-old seedlings contains a cytosolic isozyme with a pl of 5.4 ±0.1 at low specific activity (s.a, 45 ± 3 nmol min−1 mg−1 protein), found both in MT and BS. The green leaf on the other hand contains the dominant BS chloroplast isozyme with pl 4.6 ± 0.2 at a s.a, of 370 ± 40 nmol min−1 mg−1 protein (3.2 ± 0.5 μmol min−1 mg−1 chl) and a minor, previously undescribed isoform with pl 6.5 ± 0.1 also localized in the BS at a s.a. of 38 ± 6 nmol min−1 mg−1 protein. Green MT protoplasts have only traces of pl 4.6 isozyme. After illumination of dark-grown seedlings, the total leaf activity shows a rapid increase (1.5-fold within 2 h), attributed mainly to the pl 5.4 isozyme of MT protoplasts and BS strands. This is followed by a large increase of enzyme activity due to the continued rise of pl 5.4 isozyme for about 24 h and, after an initial lag of a few hours, to the accumulation of pl 4.6 isozyme. After 18 h illumination, pl 4.6 and 5.4 isozyme activities tend to decline in the MT whereas they are still increasing in the BS, particularly the former. This pl 4.6 species has become the major one by 48 h illumination. The final pattern of green leaves is established around 96 h light, when the chloroplast isozyme has attained its maximum level, the pl 5.4 isozyme of BS strands has been superceded by the pl 6.5 species (also supposed to be cytosolic) and MT protoplasts retain little residual activity. Some metabolic implications of the changing pattern of NADP-dependent malic isozymes during maize leaf greening are discussed.