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Studies on egg yolk myelin figures and granule low-density lipoproteins Garland, Timothy D.

Abstract

The exceptional emulsifying power of egg yolk has yet to be fully understood. Although it is known that certain yolk proteins and lipoprotein complexes have specific emulsifying properties, their contribution to emulsion formation and stability has not been established. Indeed, knowledge on the structure and chemical composition of some lipoproteins is lacking. This study was initiated to examine the structural, chemical and physical properties of two types of lipoprotein complexes, the myelin figure (MF) and the granule low-density lipoprotein (LDLg), and to assess the functional behavior of these components in simple oil and water emulsions. Five fractions (FI to FV) were obtained by ultra-centrifugation of washed yolk granules in 1.71M NaCl of two floating fractions, the subpellicle (FII) was found to consist of only MF and LDLg. Various techniques were employed to separate FII into fractions which were examined by electron microscopy to evaluate the homogeneity and structural integrity of the fractions. Ultrafiltration was effective for preparing native LDLg, but caused destruction of MF. Complexing the low-density lipoproteins with dextran sulfate was found to be an adequate method of separating the components of FII, but interaction of MF with dextran sulfate reduced this' method's effectiveness. Separation by Sepharose 2B gel filtration was the most rapid and effective method for separation of the two types of particles from FII. Electron microscopy of MF fraction separated by Sepharose 2B gel filtration revealed that the micelles were composed of either a single thick lamella, or two or more somewhat thinner concentric lamellae about a weakly osmiophilic core. These outer lamellae varied in electron density after OsO₄ fixation. Moreover, density gradient studies indicated that heavy and light MF species exist. Electron micrographs showed that LDLg are globular in shape, with highly electron dense surface ultraparticles. Possible structures for both MF and LDLg particles are discussed. MF and LDLg fractions, separated by Sepharose 2B gel filtration, were analyzed for total nitrogen and phosphorus by Kjeldahl and Technicon Autoanalyser methods, respectively. Nitrogen content (2.5%) of the fractions was almost identical but the phosphorus contents were 1.25% for MF and 1.05% for LDLg. The lipid components of MF and LDLg were quantitatively determined after separation by silica gel thin layer chromotography. Lipid contents of MF and LDLg were 86 and 84%, respectively. Both types of lipoprotein were composed of cholesterol, phosphatidyl choline, phosphatidyl ethanolamine, lysophosphatidyl ethanolamine, sphingomyelin, and triglycerides. Comparison of particle composition revealed that amounts of some of these constituents differed. The composition of the lipid-protein complexes of both MF and LDLg appeared to vary with age of the bird: young birds produced egg yolks whose MF and LDLg components were significantly different in total nitrogen and phosphorus contents. Electron microscopy was employed to assess .the surface characterization of emulsified oil droplets in emulsions prepared with either native and lyophilized samples of yolk, MF and LDLg. MF- and LDLg were disrupted during emulsification and the disrupted particles formed distinct zones about the surface of the oil droplets with the result of contributing to the stability of the emulsions. Three zones were observed on the surfaces of droplets of emulsions made with native MF, native LDLg, and lyophilized LDLg, whereas four zones were present on droplets in lyophilized MF emulsions. However, only one of these zones was the same in MF and LDLg emulsions. The stabilizing zones in the yolk emulsions were similar to zones in the LDLg emulsions. Since NaCl was not incorporated in the yolk emulsions, yolk granules remained intact and plasma low-density lipoproteins, similar in composition and structure to LDLg, acted as emulsifiers. Surface tension experiments showed that MF and LDLg lowered the surface tension of water to the same level as that reported for yolk solutions with the same solids content. Native LDLg emulsions were more stable than native MF and yolk emulsions when they were subjected to high speed centrifugation. Lyophilization however, of MF and LDLg emulsions, improved the stability of the resulting emulsions.

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