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The effect of milk constituents and crowding agents on amyloid fibril formation by κ-casein

Journal Article


Abstract


  • When not incorporated into the casein micelle, κ-casein, a major milk protein, rapidly forms amyloid fibrils at physiological pH and temperature. In this study, the effects of milk components (calcium, lactose, lipids, and heparan sulfate) and crowding agents on reduced and carboxymethylated (RCM) κ-casein fibril formation was investigated using far-UV circular dichroism spectroscopy, thioflavin T binding assays, and transmission electron microscopy. Longer-chain phosphatidylcholine lipids, which form the lining of milk ducts and milk fat globules, enhanced RCM κ-casein fibril formation irrespective of whether the lipids were in a monomeric or micellar state, whereas shorter-chain phospholipids and triglycerides had little effect. Heparan sulfate, a component of the milk fat globule membrane and catalyst of amyloid deposition in extracellular tissue, had little effect on the kinetics of RCM κ-casein fibril formation. Major nutritional components such as calcium and lactose also had no significant effect. Macromolecular crowding enhances protein–protein interactions, but in contrast to other fibril-forming species, the extent of RCM κ-casein fibril formation was reduced by the presence of a variety of crowding agents. These data are consistent with a mechanism of κ-casein fibril formation in which the rate-determining step is dissociation from the oligomer to give the highly amyloidogenic monomer. We conclude that the interaction of κ-casein with membrane-associated phospholipids along its secretory pathway may contribute to the development of amyloid deposits in mammary tissue. However, the formation of spherical oligomers such as casein micelles is favored over amyloid fibrils in the crowded environment of milk, within which the occurrence of amyloid fibrils is low.

Authors


  •   Liu, Jihua (external author)
  •   Dehle, Francis C. (external author)
  •   Liu, Yanqin (external author)
  •   Bahraminejad, Elmira (external author)
  •   Ecroyd, Heath
  •   Thorn, David C. (external author)
  •   Carver, John A. (external author)

Publication Date


  • 2016

Citation


  • Liu, J., Dehle, F. C., Liu, Y., Bahraminejad, E., Ecroyd, H., Thorn, D. C. & Carver, J. A. (2016). The effect of milk constituents and crowding agents on amyloid fibril formation by κ-casein. Journal of Agricultural and Food Chemistry, 64 (6), 1335-1343.

Scopus Eid


  • 2-s2.0-84958787532

Ro Metadata Url


  • http://ro.uow.edu.au/smhpapers/3717

Number Of Pages


  • 8

Start Page


  • 1335

End Page


  • 1343

Volume


  • 64

Issue


  • 6

Abstract


  • When not incorporated into the casein micelle, κ-casein, a major milk protein, rapidly forms amyloid fibrils at physiological pH and temperature. In this study, the effects of milk components (calcium, lactose, lipids, and heparan sulfate) and crowding agents on reduced and carboxymethylated (RCM) κ-casein fibril formation was investigated using far-UV circular dichroism spectroscopy, thioflavin T binding assays, and transmission electron microscopy. Longer-chain phosphatidylcholine lipids, which form the lining of milk ducts and milk fat globules, enhanced RCM κ-casein fibril formation irrespective of whether the lipids were in a monomeric or micellar state, whereas shorter-chain phospholipids and triglycerides had little effect. Heparan sulfate, a component of the milk fat globule membrane and catalyst of amyloid deposition in extracellular tissue, had little effect on the kinetics of RCM κ-casein fibril formation. Major nutritional components such as calcium and lactose also had no significant effect. Macromolecular crowding enhances protein–protein interactions, but in contrast to other fibril-forming species, the extent of RCM κ-casein fibril formation was reduced by the presence of a variety of crowding agents. These data are consistent with a mechanism of κ-casein fibril formation in which the rate-determining step is dissociation from the oligomer to give the highly amyloidogenic monomer. We conclude that the interaction of κ-casein with membrane-associated phospholipids along its secretory pathway may contribute to the development of amyloid deposits in mammary tissue. However, the formation of spherical oligomers such as casein micelles is favored over amyloid fibrils in the crowded environment of milk, within which the occurrence of amyloid fibrils is low.

Authors


  •   Liu, Jihua (external author)
  •   Dehle, Francis C. (external author)
  •   Liu, Yanqin (external author)
  •   Bahraminejad, Elmira (external author)
  •   Ecroyd, Heath
  •   Thorn, David C. (external author)
  •   Carver, John A. (external author)

Publication Date


  • 2016

Citation


  • Liu, J., Dehle, F. C., Liu, Y., Bahraminejad, E., Ecroyd, H., Thorn, D. C. & Carver, J. A. (2016). The effect of milk constituents and crowding agents on amyloid fibril formation by κ-casein. Journal of Agricultural and Food Chemistry, 64 (6), 1335-1343.

Scopus Eid


  • 2-s2.0-84958787532

Ro Metadata Url


  • http://ro.uow.edu.au/smhpapers/3717

Number Of Pages


  • 8

Start Page


  • 1335

End Page


  • 1343

Volume


  • 64

Issue


  • 6