Abstract
A Modification of Stokstad's medium for the determination of lipoic acid was carried out, with which growth of Stc. faecalis 10 Cl was more accelerated than with media reported previously. The method was also detailed which permitted the accurate turbidimetric determination of lipoic acid with Stc. faecalis 10 Cl after a 24 hous incubation period, and which had the assay range approximately 0.25 to 5.0 mμg of lipoic acid per ml. Reliability of this assay method was indicated by reproducibility of assay and by recoveries of lipoic acid added to rabbit urine and yeast extract. Growth stimulating activity of the several derivatives and analogues of lipoic acid were investigated. Methyl DL-dihydroliponate and methyl DL-6,8-diacetylthiooctanoate were found to have about 130 and 100 per cent, respec- tively, of the growth stimulating activity of lipoic acid. Smaller activities were also found for methyl DL-6-chloro-8-mercaptooctanoate and methylDL-6-hydroxy-8-mercaptooctanoate such as 16 and 1.2 per cent.
