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Isolation and characterization of mucin-like glycoprotein in human milk fat globule membrane
Abstract:
Milk fat globule membrane (MFGM) enclosing fat droplets in human milk was found to contain a high molecular weight glycoprotein which did not migrate in 10% acrylamide gel on sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). This glycoprotein (termed PAS-0) was isolated by Sepharose CL-4B chromatography. Isolated PAS-0 gave one band on SDS-PAGE using 5% acrylamide gel (acrylamide : bisacrylamide = 4 : 1, w/w) and gave one peak on analytical ultracentrifugation, indicating its homogeneity. PAS-0 was rich in serine, threonine, proline, glycine, and alanine. In contrast, contents of sulfur-containing amino acids were very low. Fucose, galactose, N-acetylglucosamine, N-acetylgalactosamine, and sialic acid were detected as constituent sugars of PAS-0 and the total carbohydrate content was about 50%. Alkali-borohydride treatment suggested that the carbohydrate moiety was linked to the polypeptide core with O-glycosidic bond(s). There results suggested that PAS-0 was a mucin-like glycoprotein. PAS-0 was shown to be resistant to pepsin, trypsin and chymotrypsin digestion, but susceptible to Pronase and Subtilisin BPN'. Extraction of intact milk fat globules (cream) with MgCl2 and guanidine hydrochloride solutions suggested that PAS-0 was an intrinsic component of MFGM. Digestion of cream with Subtilisin BPN' demonstrated that PAS-0 was located on the external surface of fat globules and was accessible to molecules outside the globules. By agglutination-inhibition tests using eight lectins, PAS-0 was suggested to act as surface receptors for Ricinus communis agglutinin, wheat germ agglutinin and peanut agglutinin.
Insights
Researchers identified a unique glycoprotein, PAS-0, within the milk fat globule membrane (MFGM). This mucin-like protein is located on the outer surface of fat globules and may function as a receptor.
Area of Science:
- Biochemistry
- Glycobiology
- Human Milk Research
Background:
- The milk fat globule membrane (MFGM) is a complex structure surrounding fat droplets in human milk.
- Understanding the composition and function of MFGM components is crucial for deciphering their biological roles.
Purpose of the Study:
- To isolate and characterize a high molecular weight glycoprotein from human MFGM.
- To investigate the structural properties, glycosylation, and location of this glycoprotein within the MFGM.
Main Methods:
- Sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) for protein analysis.
- Sepharose CL-4B chromatography for isolation.
- Analytical ultracentrifugation for homogeneity assessment.
- Amino acid and carbohydrate composition analysis.
- Enzymatic digestion (pepsin, trypsin, chymotrypsin, Pronase, Subtilisin BPN').
- Lectins agglutination-inhibition assays.
Main Results:
- A homogeneous high molecular weight glycoprotein, termed PAS-0, was isolated from human MFGM.
- PAS-0 is a mucin-like glycoprotein, rich in specific amino acids and approximately 50% carbohydrate, linked via O-glycosidic bonds.
- PAS-0 is an intrinsic MFGM component, located on the external surface of fat globules.
- PAS-0 demonstrated resistance to certain proteases but susceptibility to others, and acted as a receptor for specific lectins.
Conclusions:
- PAS-0 is a novel, mucin-like glycoprotein integral to the human MFGM.
- Its surface localization and lectin-binding properties suggest a role in mediating interactions at the fat globule surface.
- Further research into PAS-0's function could elucidate its significance in infant nutrition and development.