Introductory Remarks

Abstract

Nervous tissue proteins can be classified into five groups:

• 1) Proteins which have similar structures in most mammalian tissues, as is the case for most of the enzymes of basic metabolic pathways.

• 2) Enzymes which exist in other tissues, but which are present in different forms in the nervous system. One example of this is tyrosine α-ketoglutarate amino transferase (1, 2). This enzyme, which plays a fundamental role in tyrosine degradation probably has an important regulatory role in the nervous system since it is one of the enzymes, the other being tyrosine hydroxylase, which determines whether tyrosine is degraded or used for catecholamine biosynthesis.

  Brain tyrosine transaminase is primarily localized on the mitochondrial membrane, particularly in nerve-ending mitochondria. By contrast liver tyrosine transaminase is soluble. This attachment to the membrane may be related to the high amount of proline in the brain enzyme compared to that in liver. In addition, in brain, where the uptake of pyridoxal phosphate is not as easy as in liver, the coenzyme is strongly bound to the enzyme whereas in liver it is loosely bound. The liver enzyme has a α₄ structure, the brain enzyme α₂β₂. Both enzymes are competitively inhibited by noradrenaline. The properties of the two enzymes are compared in the table.

• 3) Enzymes involved in the biosynthesis of compounds existing only in a limited number of tissues or specific to the central nervous system, like galactolipids.

• 4) Proteins which belong to structures specific to the nervous system such as the proteins of the myelin sheath and synaptic membranes, etc.

• 5) Neuronal or glial specific proteins or enzymes of unknown function like the proteins 14.3.2 and S-100, and the enzyme 2’, 3’-cyclic AMP-3’-phosphohydrolase.