The oxidative product of syringaldazine seems to be quite stable at the temperature and pH used. Another possibility may be that your  enzyme activity is week giving a scant pink color.  So, how is about your laccase enzyme, as I am not sure if you have already characterized your enzyme for pH & temperature stability or for activity. Laccase activity is normally determined by initial reaction rate under a specified condition of temp., buffer and pH. You may try by changing your buffer or a combination of 2 buffers as some papers mentioned that laccase activity varries by changing buffer type. Best of luck.

 https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2548160/pdf/JAMMC-20-199.pdf

Thanks a lot. Its not the enzyme that is the problem, seeing that it goes a definite pink colour initially, but subsequently fades again...

Please, try to concentrate the enzyme by ammonium sulfate precipitation, if you are interesting with purification otherwise; adjust the buffer ionic strength, pH, and temp. It will work

I feel that the so called laccase that you are using could be contaminated with other enzyme. Check it and try to purify it preferably using native PAGE.

Sounds like a great idea. I was actually going to do an SDS-PAge and then incubate gel in re-naturing buffer, as I have read someone do this in a paper with a laccase. They then did an in-gel assay/zymography using guaiacol. I tried that which didn't work and was a nightmare. I want to try it  this way with the syringaldazine.

Any more tips on that? What is the thermal stability of the syringaldazine quinone? Does anyone know?

Many Thanks