Stereochemically altered noncollagenous protein from human dentin

Area of Science:

• Biochemistry
• Biomaterials Science
• Gerontology

Background:

• Highly phosphorylated noncollagenous proteins (NCPs) are key components of dentin.
• Purification of human dentin phosphoproteins is challenging due to proteolysis and diagenetic reactions.
• Racemization of aspartic acid is a known diagenetic reaction affecting proteins in vivo.

Purpose of the Study:

• To investigate the role of aspartic acid racemization in the purification difficulties of human dentin noncollagenous proteins (NCPs).
• To quantify the rate of D-aspartic acid accumulation in human dentin NCP fractions.

Main Methods:

• Purification of noncollagenous proteins (NCPs) from rat and bovine dentin.
• EDTA extraction of dentin from an age-graded series of human teeth.
• Analysis of aspartic acid racemization rates in unfractionated dentin, EDTA-soluble NCP, and EDTA-insoluble protein fractions.

Main Results:

• EDTA extraction yielded an NCP fraction with a 3x higher aspartic acid racemization rate than unfractionated dentin.
• The EDTA-soluble NCP fraction exhibited an 8x higher racemization rate compared to the EDTA-insoluble protein.
• D-Aspartic acid accumulates in EDTA-soluble NCP at 0.22% per year, reaching over 13% by age 60 in humans.

Conclusions:

• Aspartic acid racemization significantly impacts the purification of human dentin noncollagenous proteins (NCPs).
• The accumulation of D-aspartic acid in NCPs, particularly in EDTA-soluble fractions, is age-dependent and contributes to purification challenges.
• Racemization is a critical factor to consider for human dentin protein research, unlike in shorter-lived mammals.