Use of lactose [15N15N]ureide to quantify colonic salvage of urea-nitrogen

Abstract

Lactose-ureide was investigated as a potential means to deliver a known dose of ¹⁵N label directly to the colon in order to non-invasively quantify urea salvage. Two lactose-ureides, labelled with either ¹³C or ¹⁵N on the urea moiety, were synthesised and analysed for purity. Following this, a clinical trial was marginally adequate in healthy adult individuals who had accommodated to a diet marginally adequate in protein were given oral doses of the lactose-ureides, and the excretion of label was followed. Results from the lactose [¹³C]ureide administration demonstrated that little of the dose was excreted on breath before 6 h, with about 80% undergoing fermentation and hydrolysis over a 48 h period, leading to the conclusion that lactose-ureide is a suitable vehicle to non-invasively deliver label to the colon intact. An analysis of urine and stool allowed a quantification of the fate of lactose [¹⁵N¹⁵N]ureide, demonstrating that about 5% was excreted as urinary [¹⁵N¹⁵N]urea, 30% as urinary [¹⁴N¹⁵N]urea, and 22% in the stool fraction. It was concluded that, of the ¹⁵N label available to the body over half was retained on average. The discovery of label associated with lysine in the bacterial fraction of stool further suggests that de novo formed essential amino acids are available to the host. Additionally, a methodology was developed in order to directly address the potential functional significance of lysine formed de novo to the body, by attempting to separate the lysine fraction of apoliprotein B-100 hydrolysates in a quantity sufficient to determine changes in ¹⁵N enrichment using c-IRMS. An accurate detection of changes in ¹⁵N enrichment as low as 0.002 APE in samples containing 3.3 ug lysine-N was achieved, and a distinction was made between the fate of lysine, lysine-N, and lysine-¹⁵N through the analytical system.

A method has been developed which allows the salvage of urea-N to be non-invasively quantified in normal adults.

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