Abstract
This is a review of the recent chemical studies on the lipids of hen's egg yolk.
(1) The lipids in egg are concentrated in the yolk. One-third of the yolk is lipid, which consists of about two-thirds neutral lipids and one-third polar lipids.
(2) The neutral lipids are composed of sterolester, carotenoid, triglyceride, fatty acid, diglyceride, sterol, monoglyceride and so forth, among which triglyceride is predominantly much. Position 1 of the triglyceride molecules is mostly occupied by palmitic acid (saturated acid), position 2 by oleic and linoleic acids (unsaturated acids) and position 3 by oleic, palmitic and stearic acids (unsaturated and saturated acids), respectively.
(3) The polar lipids involve ceramide, cerebroside, phosphatidylethanolamine, lysophosphatidy lethanolamine, phosphatidylcholine, sphingomyelin, lysophosphatidylcholine and so on. Phosphatidylcholine and phosphatidylethanolamine are in the more amount and sphingolipids are in the less. The fatty acid composition at position 1 and position 2 of the two glycerophospholipids does not resemble to each other, and that of phosphatidylcholine and triglyceride is considerably similar. The fatty acid patterns of sphingolipids are varied to one another, although the fatty base ones are almost the same.
(4) The most of the lipids exists as lipoprotein in the yolk. The low-density lipoprotein is postulated to be an oily globule of mainly triglyceride surrounded by a thin membrane made of protein, polar lipid and probably cholesterol. The high-density lipoprotein is supposed to be a granule of protein and lipid, where a part of the lipid is rather loosely located in the surface of the protein molecule while the most of the lipid is tightly held within the network structure of the protein.
(5) In the chemical studies on the egg yolk lipids, detailed investigations of the individual lipid covering not only the major lipids but the minor lipids and attempts to clarify the existing state of the lipids in the lipoprotein molecule would be among the urgent problems.
