Pteroylpolyglutamates in human milk

Considerable variation exists in reported values for total folate content and the pteroylpolyglutamate (PteGlun) content of human milk. We investigated possible methodological sources of this variation. In two laboratories, milk folate content (with and without folate conjugase) was determined microbiologically. No differences in total milk folate or PteGlun (n > 3) content were found between laboratories. PteGlun was found to comprise a significant fraction of total milk folate (28%). Use of rennin did not alter total folate content nor the percent of PteGlun in human milk. Heating (121 °C for 5 min) increased folate concentrations (190%, P < 0.0001), indicating that release of folate from binding protein is necessary for folate utilization by Lactobacillus casei. Although human milk folate conjugase, (FC) activity was approximately one-twentieth that of plasma FC activity, it was not sufficient to autolyze endogenous PteGlun. Thus, microbiological protocols that do not use folate conjugase and do not release folate from binding proteins will seriously underestimate milk folate values.