CO2 Fixation in the Nervous Tissue

Abstract

This chapter explains CO 2 fixation in the nervous tissue. The primary CO 2 fixation reaction occurs at the oxaloacetate level. Both pyruvate carboxylase and phosphoenolpyruvate carboxykinase reactions lead to the formation of oxaloacetate, while the reaction of malic enzyme synthesizes malate. In the peripheral nerves, oxaloacetate is found to be the primary product. After CO 2 is fixed into oxaloacetate, it is transferred to all other tricarboxylic acid intermediates. The importance of CO 2 fixation at the oxaloacetate level is reflected by the 10% contribution of CO 2 fixation pathway versus the acetyl-CoA pathway in pyruvate utilization. This implies a heavy loss of carbons that are fed into the tricarboxylic acid. Glucose and pyruvate are generally considered to serve as an energy producer instead of providing carbons for other intermediates. In view of the extremely fast glutamine synthesis in the brain and the fact that glutamine is a readily diffusible compound across neuronal membrane, it is conceivable that CO 2 fixation replenishes the loss of glutamine from neuronal tissue to the blood or the cerebrospinal fluid. The chapter also discusses the functional aspects of CO 2 fixation.