Sterol carrier protein, SCP

D-bifunctional protein deficiency [5], 2-methyl acyl-CoA racemase (AMACR) deficiency [3] and sterol carrier protein (SCP-x) deficiency [6], the disorders of etherphospholipid biosynthesis (dihydroxyacetone phosphate acyltransferase and alkyl- dihydroxyacetone phosphate synthase deficiency) [2], the disorders of phytanic acid alpha-oxidation (Refsum disease) [15], and the disorders of glyoxylate detoxification with hyperoxaluria type 1 as caused by alanine glyoxylate aminotransferase deficiency as a sole representative. 

Human placental S105 has the ability to bind squalene,54 but it was not very effective in the activation of liver or placental squalene epoxidase. Although this finding does not rule out the possibility that a true sterol carrier protein (SCP) is present in placental cytosol, the lack of correlation between binding and enzyme activation makes it unlikely. 

Characteristics of sterol carrier protein, (SCP,) and sterol carrier protein2 (SCP2)... 

Metabolism of Sterols. It is interesting that a heat-stable sterol carrier protein (SCP) has been detected in the protozoan Tetrahymena pyriformis. This protozoan-SCP (P-SCP) was required, in addition to oxygen and pyridine nucleotides, for conversion of cholesterol into cholesta-5,7,22-trien-3p-ol by the protozoan microsomal enzymes (A - and A -dehydrogenase). It is interresting that both protozoan-SCP and liver-SCP are interchangeable in cholesterol biosynthesis by liver enzymes and the oxidation of cholesterol to the triene by protozoan enzymes. The effect of numerous hypocholesteraemic compounds on the cyclization of squalene to the pentacyclic triterpenoid tetrahymanol has also been studied in Tetrahymena. ... 

Both the heat-stable SCP protein of Ritter and Dempsey (R2) and the heat-labile SCP protein of Scallen et al. (S2) bind other lipids (e.g., phospholipids and fatty acids) in addition to water-insoluble cholesterol and its precursors (R2, R3, R5, S2). In view of this apparent lack of specificity, Ritter and Dempsey (R5) have suggested that the carrier protein may be more generally called lipid carrier protein (LCP), although its binding to squalene and sterol carrier protein may more directly refiect its functional role in cholesterol biosynthesis. Obviously, more work is needed to clearly define both structural role and functional properties of this protein or proteins. 

Table 3.4.1 Levels of very-long-chain fatty acids (VLCFA), pristanic acid and phytanic acid in the different peroxisomal disorders. AMACR 2-methyl acyl-CoA racemase, N normal, RCDP rhizomelic chondrodysplasia punctata, SCP-x sterol carrier protein, ZSDs Zellweger spectrum disorders,...

Fig. 8 is a schematic diagram of a cell which shows the known sites in which sterol carrier proteins are involved in cholesterol biosynthesis, utilization and intracellular transfer. SCP, participates in the conversion of squalene to lanosterol and SCP2 participates in the conversion of lanosterol to cholesterol, the conversion of cholesterol to cholesterol ester by ACAT, and probably also in the conversion of cholesterol to 7a-hydroxycholesterol. SCPj transfers cholesterol from cytoplasmic lipid inclusion droplets to mitochondria in the adrenal and SCPj also translocates cholesterol from the outer to the inner mitochondrial membrane. 

A phenanthroline ligand was also introduced to Cys specially engineered in sterol carrier protein type 2 like domain (SCP-2L), a protein that is known to bind hydrophobic molecules. In the presence of Cu(II), the hybrid enzyme catalyzed the Diels-Alder reaction depicted in Scheme 10.11 with 25% ee for the endo-isomer [70]. 

It is still difficult to reconcile completely the work of Ritter and Dempsey with that of Scallen s group on a non-catalytic carrier protein (sterol carrier protein or SCP) which is involved in the conversion of squalene into cholesterol by liver microsomes. Whereas the former workers have isolated a heat-stable protein which activates the microsomal enzymic steps for conversion of squalene into cholesterol, Scallen s group have reported a heat-labile protein with SCP properties. It has been reported that one of tlK two major apo-high-density lipoprotein peptides, apo-LP-gln II, can substitute specifically in... 

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