Endorphins are more accurately "feel good" chemicals.
Dopamine is involved in motivation through reward reinforcement, movement, and cognition in the human nervous system.
Another nitpick: L-DOPA isn't a building block of dopamine. It's dopamine with an extra molecule attached to it that makes it "inactive." This molecule gets snapped off by the AADC enzyme, which "activates" it. L-DOPA is the precursor to dopamine.
Third nitpick: We can understand how these gut flora affect ingestion of L-DOPA, and neurotransmitter precursors, by looking at how it's used in medical treatments. Most notably, Parkinson's patients. Suffice to say, it really makes no fucking difference, and orally-administered L-DOPA (with a dehydroxylaze inhibitor) is effective.
Whatever L-DOPA is stolen by gut-flora, can be augmented with a higher dosage. If for some strange reason, the converted L-DOPA -> Dopamine gets released into the digestive tract, and absorbed by the body, it's readily broken down in the blood. L-DOPA converts to dopamine in the blood-stream anyway, so it makes absolutely no difference.
I wanted this paper to be novel, but after reading it, it's not.
Gut flora have AADC-like enzymes, cool?