Cholesterol sensor

Cholesterol is the most abundant steroid in man, occurring both as free cholesterol (about 30%) and esterified with fatty acids (about 70%). The assay of cholesterol is of great importance in the diagnosis of disturbances in lipid metabolism. The normal concentration range in blood serum is 3.1-6.7 mmol/1 (Strassner, 1980). Cholesterol determination is also of importance in fermentation control and the pharmaceutical industry. 

The basis for all enzymatic cholesterol assays is the hydrolysis of cholesterol esters by cholesterol esterase (CEH, EC 3.1.1.13) to free cholesterol and fatty acids and the oxidation of free cholesterol to cholestenone by cholesterol oxidase (COD, EC 1.1.3.6) with concomitant oxygen consumption and hydrogen peroxide formation  

In the latter, the stereospecific oxidation of the 3-hydroxyl group is rate-limiting because the subsequent isomerization at the A5 position proceeds very rapidly. The reaction product, 4-cholesten-3-one, inhibits competitively (Ki = 0.13 mmol/1). 

Various COD enzymes have been isolated from microbes. Whereas extracellular COD has been shown to be an FAD enzyme, no prosthetic group could be identified in intracellular COD, e.g., from Nocardia sp. (Smith and Brooks, 1976). The molecular weight of COD is 35 000. The enzyme exhibits a broad pH optimum with maximum activity in 0.5 mol/1 

For analytical purposes cholesterol oxidase has been immobilized on various carriers (Table 6). Electrochemical, optical, and calorimetric indication have been used as detection methods. Combination of a thermistor-coupled flow-through system with immobilized COD permitted the measurement of 0.03-0.15 mmolA cholesterol (Mattiasson et al., 1976). Ogren et al. (1980) described an immobilized COD reactor for the analysis of steroid fractions obtained by high pressure liquid chromatography. The UV absorption at 240 nm of enzymatically formed cholestenone was used as the measuring signal. Linearity was found between 10 and 80 pmol/1. 

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Gimpl, G., Burger, K., Fahrenholz, F. (2002) A closer look at the cholesterol sensor. Trends Biochem. Sci. 27, 596-599. 

Kaul D, Anand PK, Verma I. Cholesterol-sensor initiates M. tuberculosis entry into human macrophages. Mol Cell Biochem 2004 258(l-2) 219-222. 

The two liver X receptors (LXRs), LXRa and LXRp, have recently been implied as drug targets for the treatment of cardiovascular diseases. Initially, LXRs were regarded as whole-body-cholesterol sensors activated by physiological concentra-... 

Urea biosensors containing urease are based on the detection of NH and HCO [7,204, 205]. Lactate dehydrogenase immobilized in PANI was used for lactate measurements [7]. Cholesterol sensors have been fabricated using choles-... 

Biosensors have also been prepared for the determination of other alcohols. For instance, a free cholesterol sensor is much more advantageous compared to the colorimetric assay of free cholesterol in serum which requires long times and many pretreatments. This was prepared from cholesterol oxidase immobilized in a collagen membrane and an oxygen electrode [232]. The oxidation of free cholesterol with dissolved oxygen by cholesterol oxidase (CHO) is given in Eq. (55). 

Figure 11. The outline scheme of a cholesterol sensor and the resultant correlation between the current observed and the amount of cholesterol linoleate.

Gomathi P, Ragupathy D, Choi JH, Yeum JH, Lee SC, Kim JC, Lee SH, Ghim HD. Fabrication of novel chitosan nanofiber/gold nanoparticles composite towards improved performance for a cholesterol sensor. Sens Actuators B. 201T,153(l) 44-9. 

With a different architecture of advanced carbon materials, graphene or graphene sheet is very attractive material to accelerate fast electron transfer process on the cholesterol sensor. As shown in Fig. 6, the sensor was composed of Pt catalyst supported by chitosan-graphene sheet. Both enzyme ChE and ChOx were physically entrapped layer by layer with Nafion polymer. A close look of morphology on the film could be observed by field emission SEM images. 

Ansari AA, Kaushik A, Solanki PR, Malhotra BD (2009) Electrochtanical cholesterol sensor based on tin oxide-chitosan nanobiocomposite film. Electroanalysis 21(8) 965-972... 

Membrane permeability based cholesterol sensor - a new possibility. J Membr Sci 164(l-2) 45 9... 

Beel AJ, Mobley CK, Kim HJ, et al. Structural studies of the transmembrane C-terminal domain of the amyloid precursor protein (APP) does APP function as a cholesterol sensor Biochemistry. 2008 47(36) 9428-9446. 

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