1-methyl-7,9-dihydro-3H-purine-2,6,8-trione 1-Methyluric acid 708-79-2 ST5298901 7,9-Dihydro-1-methyl-1H-purine-2,6,8(3H)-trione EINECS 211-905-7 C16359 1-Methyl-2,6,8-trihydroxypurine M6885_SIGMA