Formation of Methyl Sterols in Brain Cholesterol Biosynthesis

An algorithm for rapid computational construction of metabolic networks: a cholesterol biosynthesis example

Alternative pathways of metabolic networks represent the escape routes that can reduce drug efficacy and can cause severe adverse effects. In this paper we introduce a mathematical algorithm and a coding system for rapid computational construction of metabolic networks. The initial data for the algorithm are the source substrate code and the enzyme/metabolite interaction tables. The major strength of the algorithm is the adaptive coding system of the enzyme-substrate interactions. A reverse application of the algorithm is also possible, when optimisation algorithm is used to compute the enzyme/metabolite rules from the reference network structure. The coding system is user-defined and must be adapted to the studied problem. The algorithm is most effective for computation of networks that consist of metabolites with similar molecular structures. The computation of the cholesterol biosynthesis metabolic network suggests that 89 intermediates can theoretically be formed between lanosterol and cholesterol, only 20 are presently considered as cholesterol intermediates. Alternative metabolites may represent links with other metabolic networks both as precursors and metabolites of cholesterol. A possible cholesterol-by-pass pathway to bile acids metabolism through cholestanol is suggested.

Modeling cholesterol metabolism by gene expression profiling in the hippocampus

An important part of the challenge of building models of biochemical reactions is determining reaction rate constants that transform substrates into products. We present a method to derive enzymatic kinetic values from mRNA expression levels for modeling biological networks without requiring further tuning. The core metabolic reactions of cholesterol in the brain, particularly in the hippocampus, were simulated. To build the model the baseline mRNA expression levels of genes involved in cholesterol metabolism were obtained from the Allen Mouse Brain Atlas. The model is capable of replicating the trends of relative cholesterol levels in Alzheimer's and Huntington's diseases; and reliably simulated SLOS, desmosterolosis, and Dhcr14/Lbr knockout studies. A sensitivity analysis correctly uncovers the Hmgcr, Idi2 and Fdft1 sites that regulate cholesterol homeostasis. Overall, our model and methodology can be used to pinpoint key reactions, which, upon manipulation, may predict altered cholesterol levels and reveal insights into potential drug therapy targets under diseased conditions.

Lipids of nervous tissue: composition and metabolism

As indicated in the Introduction, the many significant developments in the recent past in our knowledge of the lipids of the nervous system have been collated in this article. That there is a sustained interest in this field is evident from the rather long bibliography which is itself selective. Obviously, it is not possible to summarize a review in which the chemistry, distribution and metabolism of a great variety of lipids have been discussed. However, from the progress of research, some general conclusions may be drawn. The period of discovery of new lipids in the nervous system appears to be over. All the major lipid components have been discovered and a great deal is now known about their structure and metabolism. Analytical data on the lipid composition of the CNS are available for a number of species and such data on the major areas of the brain are also at hand but information on the various subregions is meagre. Such investigations may yet provide clues to the role of lipids in brain function. Compared to CNS, information on PNS is less adequate. Further research on PNS would be worthwhile as it is amenable for experimental manipulation and complex mechanisms such as myelination can be investigated in this tissue. There are reports correlating lipid constituents with the increased complexity in the organization of the nervous system during evolution. This line of investigation may prove useful. The basic aim of research on the lipids of the nervous tissue is to unravel their functional significance. Most of the hydrophobic moieties of the nervous tissue lipids are comprised of very long chain, highly unsaturated and in some cases hydroxylated residues, and recent studies have shown that each lipid class contains characteristic molecular species. Their contribution to the properties of neural membranes such as excitability remains to be elucidated. Similarly, a large proportion of the phospholipid molecules in the myelin membrane are ethanolamine plasmalogens and their importance in this membrane is not known. It is firmly established that phosphatidylinositol and possibly polyphosphoinositides are involved with events at the synapse during impulse propagation, but their precise role in molecular terms is not clear. Gangliosides, with their structural complexity and amphipathic nature, have been implicated in a number of biological events which include cellular recognition and acting as adjuncts at receptor sites. More recently, growth promoting and neuritogenic functions have been ascribed to gangliosides. These interesting properties of gangliosides wIll undoubtedly attract greater attention in the future.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of 20,25-diazacholesterol on cholesterol synthesis in cultured chick muscle cells: a radiogas chromatographic and mass spectrometric study of the post-squalene sector

Radiogas chromatography, used in conjunction with mass spectrometry, has been used to analyze the sterol content of cultured chick muscle cells. Seven sterols, plus lanosterol, were detected. These sterols conformed to a linear biosynthetic pathway linking lanosterol and cholesterol. The reaction sequence is: C-14 demethylation, C-4 demethylation, delta 8 leads to delta 5 double bond rearrangement, delta 24 double bond reduction. When chick cells were treated with increasing concentrations of 20,25-diazacholesterol, components of this pathway and aberrant products accumulated. These accumulations suggest that diazacholesterol affects reductases, double bond isomerases and the C-14 demethylation enzymes of sterol biosynthesis.

Free sterols of the rabbit optic nerve and cerebral white matter during ontogenic development

Free sterol composition of the developing rabbit optic nerve was compared with that of the homologous cerebral white matter at corresponding stages of ontogeny. The sterols were detected and identified by means of combined gas-chromatography and mass spectrometry. The following free sterols were found in both the optic nerve and cerebral white matter: cholesterol, desmosterol, lanosterol, two dimethylsterols, which are probably 4,4-dimethyl-5 alpha-cholest-8,24-diene-3 beta-ol, with a molecular weight of 412, and 4 alpha, 14 alpha-dimethyl-5 alpha-cholest-7-ene-3 beta-ol, with a molecular weight of 414 and probably cholestene, with a molecular weight of 368. The sterol spectrum of the developing optic nerve differed not only from that of the mature nerve but also from that of age-matched white matter of the rabbit brain. The tri- and dimethyl-sterols, detected for the first time in the rabbit optic nerve and cerebral white matter, are natural components of the developing nervous tissue but they were not found in the mature nerve nor in cerebral white matter.

Effects of zuclomiphene in combination with triparanol and ay-9944 on developing rat CNS morphology and biochemistry

Developing rats were injected intraperitoneally twice weekly with a combination of three hypocholesterolemic agents: Zuclomiphene (formerly called trans-clomiphene; dosage, 30 mg/kg body weight), Triparanol (30 mg/kg body weight) and AY-9944 (3 mg/kg body weight). Treatment was initiated at 4 days of age. Biochemical and electron microscopic examination was conducted on animals sacrificed at 20 days of age. Cytoplasmic inclusion bodies were not seen in the CNS. Isolated edematous changes were seen in myelinated axons. Analysis of the sterol content of the brain and spinal cords of drug-treated animals indicated the presence of abnormal concentrations of five sterols, desmosterol, 5alpha-cholesta-7,24-dien-3beta-ol, zymosterol (5alpha-cholesta-8,24-dien-3beta-ol), 7-dehydrocholesterol (cholesta-5,7-dien-3beta-ol) and 7-dehydrodesmosterol (cholesta-5,7,24-trien-3beta-ol). Zymosterol and 5alpha-cholesta 7,24-dien-3beta-ol were minor constituents (5--7% and 1--1.5% of total sterol, respectively). The 7-dehydrosterols represented approximately one-half (44--52%) of the total CNS sterol.

Neurochemical findings in a perinatal sudanophilic leukodystrophy rich in steryl ester

Neurochemical and neuropathological studies have been made of a 10-day-old child who suffered from a sudanophilic leukodystrophy. The brain white matter contained abundant sudanophilic material. The patient's grey matter total cholesterol content was 30% higher than whole brain tissue derived from a comparable control. White matter cholesterol content was more than double the control value. Nearly 80% of the white matter cholesterol was esterified. Subcellular fractionation of the white matter resulted in a "floating fraction" rich in cholesteryl ester. The steryl ester fatty acid composition was not typical of control tissue or demyelinating tissue. Patient phospholipid fatty acid composition patterns differed from control, but white matter galactolipid fatty acid composition appeared normal. Cholesteryl ester hydrolase activity appeared normal. Myelin and myelin-like fractions, isolated from diseased and normal brain tissue, were of a primitive developing nature but appeared to be comparable. The findings indicate a neonatal sudanophilic leukodystrophy which doubtless began in prenatal life and which was rich in cholesteryl ester. The aetiology of the leukodystrophy is unknown.

Effects of trans-clomiphene in combination with AY-9944 on rat CNS morphology and biochemistry

Developing rats were injected intraperitoneally twice weekly with a combination of two hypocholesterolemic agents: trans-clomiphene 50 mg per kg body weight, and AY-9944, 5 mg per kg body weight. Treatment was initiated at 5 days of age. Biochemical and electron microscopic examination was carried out on animals sacrificed at 20 days of age. Only rarely were cytoplasmic inclusion bodies seen in the CNS. Biochemical analysis of the brain and spinal cords of treated animals indicated the abnormal accumulation of three sterols, zymosterol (5alpha-cholesta-8,24-dien-3beta-ol), 7-dehydrocholesterol (cholesta-5,7-dien-3beta-ol) and 7-dehydrodesmosterol (cholesta-5,7,24-trien-3beta-ol). The 7-dehydrosterols constituted from 56--66% of the total CNS sterol component. Zymosterol was a relatively minor (2.4--5.0%) component.