Multiple forms of glucosephosphate isomerase in maize

Area of Science:

• Plant Biochemistry
• Enzymology
• Maize Genetics

Background:

• Glucosephosphate isomerase (GPI) plays a crucial role in carbohydrate metabolism.
• Developing maize seeds contain multiple forms of GPI, necessitating characterization.

Purpose of the Study:

• To differentiate and characterize the multiple glucosephosphate isomerase enzymes present in developing maize seeds.
• To investigate the origin of multiple electrophoretic banding patterns observed for GPI I.

Main Methods:

• Column chromatography (e.g., diethylaminoethyl cellulose) was used for enzyme separation.
• Enzyme properties such as heat stability, temperature activation, and cofactor requirements were analyzed.
• Electrophoresis was employed to assess enzyme mobility and banding patterns.
• Investigated factors influencing GPI I electrophoretic heterogeneity, including dialysis, chromatography, and chemical treatments.

Main Results:

• Three distinct glucosephosphate isomerases (GPI I, II, and III) were identified in developing maize seeds.
• GPI I, found in endosperm and embryo, differs from GPI II (endosperm) and GPI III (embryo) in heat stability and zinc ion insensitivity.
• GPI II and III share similar elution profiles but differ in electrophoretic mobility and ATP response.
• Multiple banding patterns of GPI I were determined to be artifactual, influenced by purification and handling procedures, and were stabilized by mercaptoethanol.

Conclusions:

• The multiple electrophoretic forms of maize GPI I are artifacts of preparation, not distinct isoenzymes.
• GPI I and III are present in germinating seeds, while GPI II is not detected, suggesting a specialized role.
• GPI II may be specifically involved in starch biosynthesis in maize.