Incorporation of sialyl groups into a brain unidentified compound with lipophilic properties

Lactosylceramide (I), galactosylceramide (II) and lipids extracted from rat brain with methanol (III) were assayed as AcNeu acceptors from CMP-[3H] AcNeu. Rat brain microsomes in 3.2% Triton CF54 or the 100 000 g supernatant of these microsomes were used as enzyme source. For quantification of reaction products two methods were used: a) precipitation with a TCA-PTA reagent followed by extraction of the precipitate with C:M (2:1) ; b) extraction with C:M (2:1) and partition by Folch's method without salt; radioactivity was measured in the washed lower phase. No incorporation into II was detected. Using method a), incorporation into I and III was obtained. It was also found that 35% of the activity toward I and 65% of that toward III was solubilized. Using method b) of quantification, radioactivity was found in the lower phase when III was the acceptor. The product of sialylation of III was soluble in ether. Run in a column of Sephadex LH20 was eluted with chloroform in the same fraction in which a proteolipid fraction was eluted. The possibilities that the compound is either a proteolipid protein or a glycopeptide containing sialic acid are discussed.