Tandem mass spectrum similarity-based network analysis using 13C-labeled and non-labeled metabolome data to identify the biosynthesis pathway of the blood pressure-lowering asparagus metabolite asparaptine A
Asparaptine, a conjugate of arginine and asparagusic acid, was found in asparagus (Asparagus officinalis) as a naturally occurring inhibitor of angiotensin-converting enzyme (ACE) in vitro. The biosynthetic pathway to asparaptine is largely unknown; however, it is suggested that asparagusic acid may be biosynthesized from valine. To determine which metabolites are involved in the asparaptine biosynthetic pathway, we performed tandem mass spectrometry similarity-based metabolome network analysis using 13C labeled and non-labeled valine-fed asparagus calluses. We determined that valine is used as a starting material, S(2-carboxy-n-propyl)-cysteine as an intermediate, and two new metabolites as asparaptine analogs, lysine- and histidine-type conjugates, are involved in the pathway. Asparaptine was therefore renamed asparaptine A (arginine type), and the two analogs were named asparaptines B (lysine type) and C (histidine type). Oral feeding of asparaptine A to a hypertensive mouse species showed that this metabolite lowers both blood pressure and heart rate within two hours and both of which were back to normal two days later. These results suggest that asparaptine A may not only have effects as an ACE inhibitor, but also has {beta}-antagonistic effects, which are well-known to be preventive for cardiovascular diseases.