Angiotensin-converting enzyme in human skeletal muscle. A simple in vitro assay of activity in needle biopsy specimens.

The importance of skeletal muscle for human haemodynamics and metabolism makes it a suitable sample tissue for the measurement of angiotensin converting enzyme (ACE; EC 3.4.15.1). A study of serum and muscle angiotensin-converting enzyme was performed in 50 essential-hypertensive subjects. Muscle tissue was obtained from the vastus lateralis muscle by Bergström needle biopsies. The method used is a modification in Cushman's fluorometric assay for tissue angiotensin-converting enzyme. It utilized 5 mM hippuryl-histidyl-leucine as a synthetic substrate for a 60-min fixed-time incubation at 37 degrees C. The 0.25 ml incubation mixture consisted of 0.12 mM potassium phosphate buffer, pH 8.1, 300 mM NaCl, and 1 or 2 mg homogenized muscle tissue. The activity was found to have the properties previously reported for human ACE. The intra- and interassay coefficients of variation of the method were 13% and 21%, respectively, for the 1 mg incubation as assessed from internal controls (n = 20). the muscle ACE activity in the hypertensive subjects was 49 +/- 14 mU g-1 (mean +/- SD) with no difference between values for men and women. No correlation was found between muscle ACE activity and age, or between muscle and serum angiotensin-converting enzyme activity. We consider this assay a useful new tool for investigating the role of angiotensin-converting enzyme in the pathogenesis and treatment of hypertension, as it can be applied to patient materials.