let me first explain my SDS gel, the left 4 lanes ( Lane 1 to 4 ) contain individuals proteins. the right 4 lanes (lane 5 to 8) contain a mixture of the individual proteins with different buffers. the different between the buffers is the pH of Malic acid and TCEP.

lane 5: TCEP 2.5 , Malic acid 7.5

Lane 6: TCEP 2.5 , Malic acid 9

Lane7: TCEP 7.5 , Malic acid 7.5

Lane8: TCEP 7.5 , Malic acid 9

I noticed that whenever I use malic acid with pH 9 I have an intense band at 70 kDa , taking in consideration this is a contaminant coming from the original protein I couldn't get rid of it during protein purification. and that all the lanes have the same proteins concentration and loading volume.