Rapid elimination of a synthetic adjuvant peptide from the circulation after systemic administration and absence of detectable natural muramyl peptides in normal serum at current analytical limits.

Although it is clear that muramyl peptides are involved in sleep associated with bacterial infection, their role in normal physiological sleep is less certain. It has been speculated that "natural" muramyl peptides, derived from degraded gut flora, may pass into the bloodstream, where they play a role in normal sleep (M. Karnovsky, Fed. Proc. 45:2556-2560, 1986). Muramic acid serves as a chemical marker for muramyl peptides, since it is not synthesized by mammals. After injection of synthetic muramyl dipeptide in rabbits, muramic acid was readily detected (after release by acid hydrolysis) in the circulation; however, levels rapidly decreased. This was an important positive control in assessing circulating levels of natural muramyl peptides. Muramic acid was not found in normal serum (detection limit, approximately 500 pmol/ml), demonstrating the absence of appreciable amounts of circulating natural muramyl peptides. At this time we are unable to provide supportive evidence for Karnovsky's hypothesis.

Muramic acid in peripheral blood leukocytes of healthy human subjects.

Peptidoglycan, a specific marker for all bacterial cell walls, was studied in peripheral blood of healthy human subjects by mass spectrometric analysis of muramic acid. Peripheral blood mononuclear and polymorphonuclear cells from 98 healthy adults were hydrolyzed and analyzed by gas chromatography-mass spectrometry as alditol acetate derivatives using selective ion monitoring. Muramic acid was observed in cell samples from 21 of 98 subjects. Blood cultures were done simultaneously and remained negative. As a control, mononuclear and polymorphonuclear cells separated from umbilical vein blood of 41 healthy newborns were studied; all were muramic acid-negative. Since newborns lack gut flora, intestinal absorption of bacteria or of their degradation products appears to be the most likely explanation for the finding.

Muramic acid in human peripheral blood leucocytes in different age groups.

Presence of muramic acid (as a marker for bacterial cell wall peptidoglycan) was analysed by gas chromatography and mass spectrometry in the peripheral blood leucocytes of subjects from a range of ages (9-80 years) and groups (healthy individuals, patients with rheumatoid arthritis, osteoarthrosis, essential hypertension or multiple sclerosis). Sixty per cent of the sample from the youngest subjects contained detectable muramic acid. The percentage of people with circulating leucocytes containing muramic acid decreased gradually with age, being less than 5% in all groups over 40 years. No clear correlation between the presence of muramic acid and the disease was observed.