Explain the Principle of alkaline copper reduction method?

All sugars which contain a free aldehyde group undergo enolization when placed in hot alkaline solution. This property is utilized in the quantitative determination of sugars. The sugar solution is heated with alkaline copper reagent in a Folin's tube. The solution is made to react with arsenomolybdate solution to produce a blue colour. The intensity of this colour is measured colorimetrically.

For blood sugar estimation, proteins are precipitated from the sample and protein-free filtrate is heated with alkaline copper solution and reduction of alkaline copper solution follows. It is then treated with a special arsenomolybdate reagent which forms a blue colour. The colour developed is measured colorimetrically and compared with glucose standards.

To overcome some of the difficulties that arise due to the fading of colour, the more common method used for blood glucose estimation is the modification of the Nelson Somyogi method (Astoor and King, 1954 method). In this method, the reducing effect of reducing agents like glutathione are avoided by putting whole blood in isotonic solution of copper sulphate and sodium sulphate and preventing hemolysis. The red cells are not hemolysed and preserved in an unlaced condition. The glucose diffuses out of the cells while the nitrogenous reducing substances remain in the cells. They are carried along with the precipitate during deproteinization and are removed by centrifugation. The values obtained with this method are closer to true glucose values.