Generation of aspartate aminotransferase multiple forms by deamidation.

The development of aspartate aminotransferase subforms in vitro was followed by densitometry after thin-film isoelectric focusing. At the same time ammonia production was measured. Each reaction can be expressed in terms of a first-order process in which 2 mol of glutamine or asparagine/mol of dimer are deamidated with a half time of 22 days. The more negatively charged subforms developed in vitro were almost fully active. Another process occurred leading to inactivation by coenzyme modification, and this was independent of deamidation. Although the enzyme formed absorbed maximally at 340nm, it was different from the naturally occurring inactive enzyme that absorbs at this wavelength.     

LEAF CAVITY SIZE DIFFERENTIATION AND WATER RELATIONS IN CAREX ELEOCHARIS

Carex eleocharis leaves contain large intercellular cavities that traverse the length of the leaf above rows of stomata and are roughly constant in size throughout a leaf. Semithin sections (1–2 μm) demonstrate that the substomatal chambers are directly continuous with the intercellular cavities. Leaves of plants inhabiting moist swale regions of the shortgrass steppe, in northeastern Colorado, were found to have larger cavities as compared to leaves of plants inhabiting dry hilltops. Plants collected from a common hilltop site were grown in a controlled environment chamber, and, by manipulating the watering schedule, we obtained water potentials similar to those in the field. Leaves of “well-watered” plants were found to have larger intercellular cavities as compared to “water-stressed” plants. Leaf mesophyll cell sizes did not differ significantly between “well-watered” and “water-stressed” plants, suggesting that cavity size differences are not the result of developmental differences. Leaf cavities were shown to contain gases and to occur along the leaf length above rows of stomata. Additionally, the cavities in unstressed plants were continuous with substomal chambers. It is proposed that the reduction of cavity size is a mechanism to reduce water loss from the leaves during periods of plant water stress.