8-Demethyl-6-methylriboflavin; a riboflavin antimetabolite, differing from riboflavin in that the methyl groups on the isoalloxazine nucleus are in the 6,7 positions rather than the 7,8.
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Liquid-chromatographic measurement of riboflavin in serum and urine with isoriboflavin as internal standard.
Briefly, 50 [micro]L of isoriboflavin (internal standard) was added to 300 [micro]L of plasma and calibrators.
We appreciate the donation of the isoriboflavin internal standard by Dr.
Plasma samples (500 [micro]L) were mixed with 1500 [micro]L of ice-cold 100 g/L TCA containing 15 nmol/L isoriboflavin, which served as an internal standard.
Isoriboflavin is an isomeric form of riboflavin (8-demethyl-6-methylriboflavin) and has been used previously as an internal standard in HPLC assays (15).
We investigated the stability of the vitamers and the internal standard in TCA-treated plasma before neutralization by adding 500 nmol/L riboflavin, FMN, FAD, or isoriboflavin to the samples.
To study the stability of the internal standard in TCA used for plasma protein precipitation, we studied aliquots of 100 g/L TCA containing 150 nmol/L isoriboflavin incubated for 1, 2, 4, 7, and 14 days.
Briefly, a 40-[micro]L plasma sample was mixed with 400 [micro]L of trichloroacetic acid (100 g/L) containing 15 nmol/L isoriboflavin (internal standard), and 330 [micro]L of the supernatant was neutralized by the addition of 108 [micro]L of [K.
Trichloroacetic acid (75 [micro]L of a 100 g/L solution) containing 15 nmol/L isoriboflavin was added to the samples, and 75 [micro]L of the supernatant was neutralized by the addition of 24 [micro]L of 2 mol/L [K.