Lisinopril works by directly inhibiting the action upon which substance?

Lisinopril is an ACE inhibitor: it blocks the angiotensin-converting enzyme that normally converts angiotensin I into angiotensin II. By blocking this specific conversion step, lisinopril directly prevents angiotensin I from being acted upon/converted — ultimately lowering downstream angiotensin II and aldosterone levels indirectly, but the direct point of action is on the conversion of angiotensin I.