The role of dietary lactose in the synthesis of brain galactolipids

Abstract

The brain is endowed with considerable autonomy in the synthesis of myelin lipids, several components of which can be made in 8itu, e.g. cholesterol, sphingosine and fatty acids. Myelination takes place at different stages of ontogenetic development, being virtually complete at birth in some species, or beginning only after birth in others. Cerebroside, one of the myelin lipids, appears relatively late in the phylogenetic evolution of the brain. None has been found in the central nervous system of some invertebrates (squid, arachnoid king crab, honey bee; McColl & Rossiter, 1950; Patterson, Dumm & Richards, 1945) and some of the positive reports of the occurrence of cerebrosides in crustaceans, fishes, amphibians, birds and even neonatal mammals are suspect on account of inadequate analytical techniques. Estimation of cerebrosides, based on determinations of total carbohydrate by relatively non-specific methods, often yields high results due, in part, to the presence of lipid-bound glycogen (S. N. Varma & V. Schwarz, unpublished work), gangliosides and possibly other glycolipids. Questionable as they are, the meagre data on the cerebroside content of the brain of different species suggest, however, that there is a gradual increase with evolution and ontogenetic development (Lanfranchi, 1938; McColl & Rossiter, 1952). The occurrence of lactose in the milk of all mammals examined so far has engendered speculation on a possible connexion between galactolipid synthesis and dietary galactose. Ziegler (1960) has drawn attention to a rough correlation between the lactose content of milk and the degree of brain development in different species. Thus human, horse and elephant milk has a lactose content of about 7%, cow, sheep and cat milk about 4.5%, and porpoise, guinea pig and rabbit milk about 2%. A crude proportionality between milk lactose and brain galactolipid has been observed by Sadhu (1948), who also found the cerebroside content of pigeon and rat brain to be increased after feeding with relatively large amounts of lactose. Of the two known galactose-containing families of lipids, gangliosides occur almost exclusively in the grey matter and (in man) are present at birth