B727 DSNDS BELOW CLRED ALT IN DEN AIRSPACE.

Narrative

APCHING DEN ON THE TOMSN 1 STAR; DSNDING TO INTERCEPT THE FALCON (FQF) 306 DEG RADIAL AND CROSS TOMSN INTXN AT FL190 -- IN AND OUT OF CLOUDS. IMMEDIATELY PRIOR TO INTERCEPTING THE FQF 306 DEG RADIAL; WE NOTICED WE WERE BORE-SIGHTED ON AND RAPIDLY OVERTAKING A TWIN TURBOPROP IMMEDIATELY BENEATH (PERHAPS 1500 FT BELOW). WHILE THERE WAS NO CONFLICT (TCASII OR OTHERWISE) IT WAS; NONETHELESS; DISCONCERTING TO BE TURNING; DSNDING AND ATTEMPTING TO MAKE A XING RESTR IN A FEW MI; AND HAVE A COMMUTER POP UP DIRECTLY AHEAD OF AND IMMEDIATELY BELOW US. NO MENTION WAS MADE OF THE ACFT BY ATC. IMMEDIATELY THEREAFTER WE LEVELED AT FL190; CROSSED TOMSN INTXN AND WERE ADVISED TO EXPECT RWY 7 AT DEN (A RWY ON WHICH I HAD NEVER LANDED). WHILE THE FO AND SO KEPT ME APPRISED OF THE WHEREABOUTS OF THE TURBOPROP (HE CROSSED L TO R BENEATH US); I BUSIED MYSELF WITH GETTING OUT THE APCH PLATE AND BRIEFING THE APCH TO RWY 7. I BELIEVED WE WERE CLOSE IN AND WOULD NEED TO GET DOWN QUICKLY FROM FL190 TO COMPLETE THE APCH. UNFORTUNATELY; GIVEN THE PERCEIVED IMMEDIACY OF THE SIT; I MISSED MANUALLY ENGAGING THE ALT HOLD FEATURE OF THE AUTOPLT. THE ACFT BEGAN A SLOW DSCNT AND BY THE TIME THE COPLT AND SO NOTICED THE DEV; AND I HAD RETURNED THE ACFT TO FL190; WE HAD LOST APPROX 400 FT. ATC QUERIED US ABOUT THE DSCNT BUT MENTIONED NOTHING ABOUT A CONFLICT WITH ANY OTHER ACFT. THERE WERE NO TCASII ALERTS; NOR DID THE ALT ALERTER SOUND -- WHICH WOULD INDICATE A 500 FT DEV.