Phospholipid biosynthetic enzymes in human brain

Abstract

Growing evidence suggests an involvement of brain membrane phospholipid metabolism in a variety of neurodegenerative and psychiatric conditions. This has prompted the use of drugs (e.g., CDPcholine) aimed at elevating the rate of neural membrane synthesis. However, no information is available regarding the human brain enzymes of phospholipid synthesis which these drugs affect. Thus, the objective of our study was to characterize the enzymes involved, in particular, whether differences existed in the relative affinity of substrates for the enzymes of phosphatidylethanolamine (PE) compared to those of phosphatidylcholine (PC) synthesis. The concentration of choline in rapidly frozen human brain biopsies ranged from 32-186 nmol/g tissue, a concentration similar to that determined previously for ethanolamine. Since human brain ethanolamine kinase possessed a much lower affinity for ethanolamine (Km = 460 microM) than choline kinase did for choline (Km = 17 microM), the activity of ethanolamine kinase in vivo may be more dependent on substrate availability than that of choline kinase. In addition, whereas ethanolamine kinase was inhibited by choline, and to a lesser extent by phosphocholine, choline kinase activity was unaffected by the presence of ethanolamine, or phosphoethanolamine, and only weakly inhibited by phosphocholine. Phosphoethanolamine cytidylyltransferase (PECT) and phosphocholine cytidylyltransferase (PCCT) also displayed dissimilar characteristics, with PECT and PCCT being located predominantly in the cytosolic and particulate fractions, respectively. Both PECT and PCCT exhibited a low affinity for CTP (Km approximately 1.2 mM), suggesting that the activities of these enzymes, and by implication, the rate of phospholipid synthesis, are highly dependent upon the cellular concentration of CTP. In conclusion our data indicate different regulatory properties of PE and PC synthesis in human brain, and suggest that the rate of PE synthesis may be more dependent upon substrate (ethanolamine) availability than that of PC synthesis.

Oral administration of circulating precursors for membrane phosphatides can promote the synthesis of new brain synapses

“…If similar increases occur in human brain, giving these compounds to patients with diseases -like Alzheimer's Disease -which cause the loss of brain synapses could be beneficial. Phosphatidylcholine (PC) biosynthesis via the Kennedy cycle [17]. In rats, cytidine is the major circulating pyrimidine [95]; in humans [19] and gerbils [20] the primary circulating pyrimidine is uridine.…”

“…The phosphorylation state of CT affects its net activity [45], as does its substrate saturation with CTP and perhaps with phosphocholine. The Km's of CT for CTP and phosphocholine in brains of laboratory rodents and humans are reportedly 1-1.3 mM and 0.30-0.31 mM [17,46], respectively, while brain levels of these compounds are only 70-110 μM [11,47,48] and 0.32-0.69 mM [12,16,49] respectively. Hence, brain CT normally is highly unsaturated with CTP, and only about half-saturated with phosphocholine in vivo, suggesting that its degrees of substrate-saturation, particularly with CTP, exert important limiting roles in PC synthesis.…”

“…Like CK, ChAT has a very low affinity for its choline substrate [25,26]. The Km's of these enzymes in brain (which, of course, describe the choline concentrations at which the enzymes operate at only half-maximal velocity) are reportedly 2.6 mM [14] and 540 μM [27], respectively, whereas brain choline levels are only about 30-60 μM [15][16][17]. Hence, the syntheses of both phosphocholine and ACh are highly responsive to treatments which raise or lower brain choline levels.…”

“…Thus, choline administration increases brain phosphocholine levels in rats [12] and humans [13], because CK's Km for choline (2.6 mM [14]) is much higher than usual brain choline levels (30-60 μM) [15][16][17]. Most commonly the second, CT-catalyzed reaction is most ratelimiting in PC synthesis, either because not all of the CT enzyme is fully activated by being attached to a cellular membrane [18] or because local CTP concentrations are insufficient to saturate the CT [17].…”

“…Most commonly the second, CT-catalyzed reaction is most ratelimiting in PC synthesis, either because not all of the CT enzyme is fully activated by being attached to a cellular membrane [18] or because local CTP concentrations are insufficient to saturate the CT [17]. Thus, when brain CTP levels are increased by giving animals uridine [11], CTP's circulating precursor in human blood [10], PC synthesis is accelerated [11].…”