Lactose, colorimetric determination

Lactose is also found bound to JV,0-diaretylneuraminic acid, which is removed by weak acids or by neuraminidase . The concentration of this trisaccharide, neuraminosyl-lactose (O-acetyl-lactaminic acid-lactose) in human milk, based on the colorimetric determination , is 50-60 mg %. This amount, however, probably includes other oligosaccharides which contain neuraminic acid. The reducing group of the lactose moiety is free. A study... 

Because of their speed and reasonable instrumental requirements, there is continuing interest in colorimetric procedures for lactose determinations. Most procedures are based on the reducing properties of lactose samples that contain only one sugar are easily measured colori-metrically, but samples with three or four sugars may require several different colorimetric assays to determine the composition accurately. 

Wahba, N. 1965. A simple micro colorimetric method for the determination of lactose in milk. Analyst 90, 432-434. 

Enzymes modified with carbohydrates (neoglycoenzymes) can be used in cytochemistry as described above or in biochemical detection of lectins in solid-phase assays to gain greater sensitivity in analysis. For example, bacterial 3-galactosidase modified with p-aminophenyl a-D-mannopyranoside via amide linkage was useful in determination of Con A immobilized on plastic microtiter plates, and lactose-modified P-galactosidase was effective in histochemical detection of galactoside-specific lectins [63]. Other enzymes frequently used for these applications are alkaline phosphatase and horse radish peroxidase. There are a number of colorimetric, fluorometric, and chemiluminescent substrates available for these enzymes. 

The formaldehyde formed in a periodate reaction is determined gravimet-rically with Dimedon, or colorimetrically with chromotropic acid, after the excess oxidant is destroyed with ethylene glycol, stannous chloride, sodium arsenite, or sodium bisulfite 221), The formic acid is generally titrated directly. Titration to pH 6.0 will measure all the formic acid present. Titration to higher alkalinities will usually measure any formic acid bound as an ester 222) but also other acidic products the end-point may be indefinite. Sucrose will give 80 to 90 % of the theoretical formic acid, and maltose and lactose similarly low values. Both formaldehyde and formic acid are attacked only extremely slowly by periodate, but light apparently accelerates an oxidation to carbon dioxide and water 223),... 

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