Keratan sulphate--a 'reserve' polysaccharide?

The early history of keratan sulphate and its proteoglycans is briefly described. Studies were overlooked that could have had a profound influence on later work. Early methods of writing the structures of keratan and chondroitin sulphates obscured the fundamental relationships between them. Both are now seen to be based on the same polymer backbone poly(Gal beta 1:4 Glc beta 1-3). Confusion over the complicated sulphation patterns in keratan sulphate was clarified by the domain structure idea by the group of Helmut Greiling. Keratan sulphate is characteristic of avascular tissues (cartilages, intervertebral discs, corneal stromas) that get their oxygen supplies by diffusion. Stockwell's early idea that the distribution of keratan sulphate in cartilages was a response to the poor supply of oxygen has been generalised, to the hypothesis that keratan sulphate is a functional substitute for chondroitin sulphate under conditions of oxygen lack. The keratan:chondroitin sulphate ratios in discs, corneas of different species, and changes therein with age can be explained on this basis. The biochemical controlling step is probably the NAD:NADH ratio. Keratan sulphate may thus be a 'reserve' polysaccharide, able to do the job of chondroitin sulphate in adverse conditions of oxygen supply. Keratan and chondroitin/dermatan sulphates have similar functions in corneal stroma, and probably in the other connective tissues in which they are found. They swell the collagenous matrix, keeping the fibrils apart. Even more importantly, they probably act as tissue organisers, orienting the fibrils vis-a-vis each other via specific interactions of their proteoglycan protein cores with the fibrils.(ABSTRACT TRUNCATED AT 250 WORDS)