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Esterified milk proteins inhibit DNA replication in vitro
M Sitohy1, J M Chobert, J C Gaudin
1Laboratoire d'Etude des Interactions des Molécules Alimentaires, Institut National de la Recherche Agronomique, B.P. 71627, 44316 Cedex 3, Nantes, France.
Abstract:
DNA replication was studied in vitro in the presence of native and esterified milk proteins [alpha-lactalbumin (ALA), beta-lactoglobulin (BLG) and beta-casein (BCN)]. Addition of unmodified proteins to the PCR medium did not change the result of the reaction seen by electrophoresis, even at excessive ratios of basic amino acids in proteins:phosphate groups in DNA as high as 100:1. Addition of esterified proteins greatly reduced the intensity of the bands corresponding to the newly synthesized DNA, at ratios as low as 1:1 and 5:1 in case of methylated-BLG and methylated-ALA, respectively. The inhibitory effect of esterified proteins was directly proportional to their extent of esterification and strongly related to their DNA-binding capacity. Generally, inhibition of PCR with esterified proteins was similar to what can be observed with histones. However, stronger inhibition was observed with highly esterified proteins when using a higher ratio of basic:acid residues (1:1) when compared with 0.5:1 ratio in case of histones. Highly esterified BCN did not exert any inhibitory effect because of its relatively lower pI when compared with that of other esterified milk proteins and due to its lower positive net charge at the pH used for PCR. During a second PCR run, only the addition of new DNA template was able to reinitiate the reaction, giving rise to new synthesized DNA. Addition of Taq DNA polymerase did not enhance DNA synthesis, showing that inhibition was performed only by binding of DNA template and not by the inhibition of the polymerase.
Insights
Esterified milk proteins inhibit DNA replication in vitro by binding to DNA, similar to histones. Unmodified proteins showed no inhibitory effect on the PCR reaction.
Area of Science:
- Biochemistry
- Molecular Biology
- Food Science
Background:
- Milk proteins are common food components.
- DNA replication is a fundamental biological process.
- Understanding interactions between proteins and DNA is crucial.
Purpose of the Study:
- To investigate the effect of native and esterified milk proteins on DNA replication in vitro.
- To compare the inhibitory potential of modified milk proteins with histones.
- To elucidate the mechanism of inhibition during Polymerase Chain Reaction (PCR).
Main Methods:
- In vitro DNA replication assays using PCR.
- Analysis of synthesized DNA by gel electrophoresis.
- Varying ratios of milk proteins to DNA.
- Comparison with histone effects.
Main Results:
- Esterified milk proteins (methylated-BLG, methylated-ALA) significantly inhibited DNA synthesis.
- Inhibition was proportional to the extent of esterification and DNA-binding capacity.
- Esterified milk proteins showed histone-like inhibition, with higher inhibition at increased basic:acid residue ratios.
- Highly esterified beta-casein (BCN) showed no inhibition due to lower pI and net positive charge.
- Inhibition was due to DNA binding, not polymerase inhibition, as only new DNA template reinitiated the reaction.
Conclusions:
- Esterified milk proteins, particularly methylated alpha-lactalbumin and beta-lactoglobulin, can inhibit DNA replication in vitro.
- The inhibitory mechanism involves binding to the DNA template, similar to histones.
- The charge and esterification level of milk proteins influence their DNA-binding and inhibitory capacity.