G. Brian Ansell

The metabolism of choline in regions of rat brain and the effect of hemicholinium-3

Abstract When [ Me - 14 C]choline was injected intracerebrally, the radioactivity rapidly spread throughout the brain but the distribution in the six regions examined was not uniform. HC-3 caused a higher retention of the labelled choline particularly in the side of the brain which was the site of the injection. In the cerebellum and mid-brain the synthesis of phosphorylcholine was inhibited by HC-3 but the synthesis of phosphatidylcholine was stimulated. This may be due to an increase in base exchange in the presence of HC-3. In the cortex, phosphorylcholine synthesis was inhibited by HC-3 but no stimulation of phosphatidylcholine synthesis was found. This regional difference may demonstrate that the pathways for the formation of phosphatidylcholine are not the same throughout the brain.

Functional Metabolism of Brain Phospholipids

Publisher Summary This chapter presents the recent developments in which metabolic processes involving phospholipids may be related to mechanisms important in the functioning of the brain. The chapter focuses on the phospholipid composition and metabolism of relatively homogeneous structural components of the nervous system, for example, myelin and the plasma membranes of nerve endings, which are more likely to yield useful information and have, in fact, done so. However, it is not clear why different membranous components of the nervous system have a different phospholipid composition. This comment applies, of course, to other tissues and the rationale for different phospholipid species is often totally obscure. One certainty is that some membrane-bound enzymes need specific phospholipids in order to function normally. When the metabolism of a phospholipid is abnormal then the function can be seriously disturbed. In this way, an abnormal metabolism gives a clue to a normal function, as it has done for other tissue components.

THE RELEASE OF FREE ETHANOLAMINE IN RAT BRAIN HOMOGENATES INCUBATED IN KREBS RINGER

Ethanolamine in mammalian brain is found chiefly in a lipid-bound form either as the diacyl phospholipid, phosphatidylethanolamine or as the plasmalogen, I-alk-11-enyl-2-acyl glycerophosphoethanolamine and to a lesser extent as the saturated ether analogue. The level of free ethanolamine in brain is very low, probably less than 40 nmol/g brain (Spanner & Ansell, 1977b) while that of phosphoethanolamine is about 1.0 mumol/g brain. Some time ago we found that if brain tissue was incubated in Krebs Ringer bicarbonate (pH 7.4) at 37 degrees, there was a steady release of free ethanolamine by the tissue. The following account is a summary of the findings from experiments designed to determine the source of the ethanolamine liberated.

The subcellular fractionation of the bovine caudate nucleus

Two synaptosomal fractions could be obtained from bovine caudate nucleus on sucrose density gradients one of which had a much greater capacity for ‘high affinity’ choline uptake than the other but comparable amounts of CAT and choline kinase activity. Specific binding of QNB was widely distributed among all the subcellular fractions except the mitochondrial fraction and in quantitative terms by far the greatest amount was in the microsomal fraction. Only the microsomal fraction contained measurable amounts of glycerophosphocholine phosphodiesterase.

The Role of Choline Kinase in the Brain

Choline kinase (EC 2.7.1.2) was first demonstrated in yeast by WITTENBERG & KORNBERG (1953) and, in spite of the obvious importance of choline phosphorylation as the first step by which choline is incorporated into phosphatidylcholine by the cytidine pathway, it has not been greatly investigated. Phosphorylcholine was first demonstrated in brain tissue by DAWSON (1955) and choline kinase first studied in detail by McCAMAN (1962) who showed that its activity was higher in brain tissue than other mammalian tissues and that it was preferentially associated with grey matter e.g. the molecular layer of the cortex. McCAMAN showed that it depended on Mg2+ ions for activity, had a pH optimum of about 9 and a Km for choline of 5 x 10-3M. In 1966 McCAMAN & COOK extended these observations and showed that the enzyme was mainly associated with the soluble fraction of brain tissue and this was confirmed by ANSELL & SPANNER (1972).

Synergistic enhancement of histamine release from rat peritoneal mast cells by the phorbol ester 12-O-tetradecanoylphorbol 13-acetate is not reflected by corresponding changes in phospholipid turnover

In an attempt to elucidate further the relationship between changes in phospholipid metabolism in, and histamine secretion from, purified rat peritoneal mast cells, the effects of the phorbol diester 12-O-tetradecanoylphorbol 13-acetate (TPA) on these responses in stimulated and unstimulated cells was investigated. TPA caused a dose-dependent increase in the incorporation of 32PO4(3-) into the mast cell phospholipids; phosphatidic acid (PA) and phosphatidylcholine (PC), but not phosphatidylinositol (PI). TPA synergistically enhanced histamine release from cells stimulated by anti-immunoglobulin E (IgE) and the calcium ionophore A23187, reducing its ED50 from 150 nM to 40 nM, but did not alter histamine release from cells stimulated by compound 48/80. The effect of TPA on the changes in 32PO4(3-) incorporation into phospholipids associated with the above secretagogues did not, however, correlate well with the observed effects on histamine secretion induced by the same secretagogues. These observations are discussed in relation to the known effects of phorbol esters upon both secretory processes and phospholipid metabolism in other tissues.