Reference Intervals for the Kinetic Anglotensin Converting Enzyme

Buttery (1) asks why, with use of basically the same method [furylacryloyl-L-phenylalanylglycylglycine (FAPOG) as substrate], there are differences in the published reference intervals for angiotensin converting enzyme (ACE, EC 3.4.15.1). In particular, he cites differences between ours (2) and that of Neels et al. (3) and asks whether an incorrect differential millimolar absorptivity (the difference in absorbance between 1 mmol/L solutions of substrate and product, referred to as “M” by Buttery) is responsible. The differential absorptivity of FAPGG changes greatly with respect to wavelength and, as pointed out (4), may, even at the same nominal wavelength, be different in different instruments depending on the particular characteristics, such as the bandpass of the instrument.We measured the differential absorptivity at the wavelength and with the instrument we used to do the analysesand are therefore sure that the correct differential absorptivity was used. What, then, could explain the differences? The differences in the reference intervals are, as pointed out (2), at least partly explicable on the basis of the different substrate concentrations used (0.9 mmol/L as opposed to 0.75 mniol/L). When FAPGG is used as substrate in ACE measurement, it is not possible to use concentrations greatly in excess of its Km (0.3 mmol/L), because to do so would lead to unmeasurably high absorbances. At substrate concentrations not greatly in excess of the Km for an enzyme, the activity of the enzyme is substrate dependent. Buttery found no effect of substrate concentration on ACE activity. This contrasts with our findings (2) and those of Ronca-Testoni (4). We agree that the situation where there are different reference intervals is not desirable. The answer is to agree on standard conditions for ACE measurement with FAPGG as substrate. We suggest using a substrate concentration of 0.8 mmolJL, a wavelength of 340 nm, and with the correct differential absorptivity being established for each instrument.