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Heat-induced fragmentation of human alpha 2-macroglobulin
Abstract:
Previous studies have demonstrated that human plasma alpha 2-macroglobulin (alpha 2 M) possesses a single subunit chain (Mr approximately 185,000) when incubated with dodecyl sulfate and dithiothreitol at 37 degrees C and analyzed by dodecyl sulfate-gel electrophoresis. The present study details the observation that heating alpha 2 M to 90 degrees C under identical conditions produces at least two additional polypeptide chains, termed bands II and III, with apparent molecular weights of 125,00 and 62,000. The generation of these fragments is enhanced by increasing the time of incubation. The appearance of band II composition of the buffer, dodecyl sulfate concentrations, or alpha 2 M protein concentration in the incubation mixture. The electrophoretic bands II and III of alpha 2 M have dissimilar 125I-labeled tryptic peptide digests and also differ in their amino acid composition. The heat-induced fragmentation of alpha 2M is not affected by the inclusion of a variety of low molecular weight protease inhibitors, suggesting that the appearance of bands II and III is not due to enzyme-catalyzed hydrolysis. When the subunit chain of alpha 2M is first cleaved by trypsin into the previously described Mr = 85,000 derivative, neither band II nor III material, nor other lower molecular weight products are generated by heat treatment. Furthermore, preincubation of alpha 2M with methylamine prevents fragmentation of the subunit chain. These results indicate that these fragments are neither pre-existing subunits of alpha 2M nor derivatives formed prior to treatment for gel analysis. These data provide evidence that a covalent bond in the alpha 2M molecule is unusually susceptible to heat-induced cleavage.
Insights
Heating human plasma alpha 2-macroglobulin (alpha 2 M) to 90°C causes fragmentation into new polypeptide chains. This heat-induced cleavage reveals an unusual susceptibility to heat, not caused by enzymatic activity.
Area of Science:
- Biochemistry
- Molecular Biology
- Protein Chemistry
Background:
- Human plasma alpha 2-macroglobulin (alpha 2 M) is known to exist as a single subunit chain under standard denaturing conditions.
- Previous research utilized dodecyl sulfate-gel electrophoresis to characterize alpha 2 M.
Purpose of the Study:
- To investigate the effect of elevated temperatures on the structural integrity of alpha 2 M.
- To identify any novel polypeptide fragments generated from alpha 2 M under heat stress.
Main Methods:
- Incubation of alpha 2 M with dodecyl sulfate and dithiothreitol at 90°C.
- Analysis of resulting polypeptide chains using dodecyl sulfate-gel electrophoresis.
- Characterization of fragments through 125I-labeled tryptic peptide digests and amino acid composition analysis.
Main Results:
- Heating alpha 2 M to 90°C produced two additional polypeptide chains (bands II and III) with molecular weights of 125,000 and 62,000.
- Fragment generation increased with incubation time and was independent of buffer composition, dodecyl sulfate concentration, or protein concentration.
- Tryptic digests and amino acid compositions of bands II and III differed, and heat-induced fragmentation was not inhibited by protease inhibitors.
- Pre-cleavage with trypsin or pre-incubation with methylamine prevented heat-induced fragmentation.
Conclusions:
- The heat-induced fragmentation of alpha 2 M is not due to enzymatic hydrolysis or pre-existing subunits.
- These findings indicate a specific covalent bond within the alpha 2 M molecule is highly susceptible to heat-induced cleavage.