Composition

PSK is a light or dark brown odourless and tasteless powder which readily dissolves in water. It contains approximately 35% carbohydrate (91% beta-glucan), 35% protein and the remainder are free residues such as sugars, amino acids and moisture.6

PSK is a mixture of polysaccharides covalently linked to various peptides with an average molecular weight of 100 kilodaltons.7 The polysaccharide component is in a class of beta-glucans which comprise of glucopyranose units (Fig. 1). Structural analysis showed that PSK has a 1, 4–glucan configuration as the main glucoside portion with branches at positions 3 and 6 at a frequency of one branch per several residual groups of 1 – 4 bonds (Fig. 1).

Fig. 1.  Structure of the sugar portion of PSK

The major constituent is a monosaccharide (glucoside) with lesser amounts of other saccharides (Table 1)

Table 1. Monosaccharide composition of PSK7

 

MonosaccharideComposition (%)
Glucose74.6
Mannose15.5
Xylose4.8
Galactose2.7
Fucose2.4

The protein portion of PSK consists mainly of acidic amino acids, namely aspartic acid and glutamic acid and also neutral amino acids including valine and leucine and small amounts of lysine and arginine7 (Table 2). This basic glucan structure and its high water solubility are probably responsible for the high anti-tumour activities of PSK.

Table 2.   Amino acid composition of PSK7

Amino AcidComposition (%)Amino AcidComposition (%)
Glutamic acid14.4Threonine4.5
Aspartic acid13.2Tyrosine2.9
Leucine13.4Lysine2.8
Alanine9.2Histidine2.3
Valine9.6Methionine1.9
Glycine7.8Arginine0.7
Phenylalanine6.7ProlineTrace
Isoleucine5.9CystineTrace
Serine4.7TryptophaneTrace