A 62-year-old woman with Parkinson's disease of 8 years' duration is on levodopa-carbidopa and experiences severe morning rigidity and "off" episodes lasting 2–3 hours. Her neurologist adds entacapone to her regimen. Within 2 weeks, her "off" time reduces to 30 minutes, and her motor fluctuations improve significantly. Which of the following best explains the clinical benefit of adding entacapone?