A nitrogen balance experiment using simulated urine samples

Abstract

We describe an undergraduate laboratory experiment that combines the advantages of problem-based learning with the need for biochemistry students to become proficient in practical laboratory skills. It also avoids the need to obtain ethical approval for recruiting volunteers and eliminates any possible biosafety issues with the handling and disposal of large amounts of urine. Simulated human urine samples are prepared that contain urea, uric acid, and creatinine at concentrations that represent the levels expected in 2 liters of urine collected over 24 h from subjects on various protein diets or during different physiological states. The students measure the nitrogen-containing compounds in the "urine samples" using specific colorimetric assays and use the data they generate to derive knowledge about nitrogen balance and the excretory metabolism of amino acids.Keywords: Nitrogen balance, urea, uric acid, creatinine.The introduction of problem-based learning into many biochemistry courses has added an important means of encouraging active higher level learning [1]. However it has often meant that the wet-lab, hands-on, practical component of biochemistry is neglected. Many courses that cater to students in the health related professions do not include practical laboratory sessions at all and have chosen to replace them with paper-based exercises or problems. When ready-made data are provided for these problems, students lose the appreciation of how the data are actually obtained and of course become less proficient in standard laboratory skills. An important goal of biochemical training is to learn how to pose questions, plan and carry out experiments, and obtain, analyze, and interpret the data. For this reason we have deliberately maintained practical laboratory sessions for all of our courses, although the time spent on doing practical exercises has been reduced in favor of developing project-style sessions, which allow the students more time for planning, thinking, and discussion.We teach a one-semester course on human metabolism. Typically the students who enroll for this course are in their second year of a science degree program, majoring in biochemistry, nutrition, or physiology, or are in their second year of an undergraduate professional degree in medicine, dentistry, pharmacy or medical laboratory technology, in which biochemistry is a requirement. All these students have completed one-semester introductory courses in general biology and chemistry as well as a foundational course in biochemistry, which covers the structure and function of proteins and outlines the main catabolic pathways of carbohydrates and lipids. The students become familiar with the vocabulary and basic concepts of these sciences but get only limited laboratory experience.The practical sessions of the human metabolism course focus on aspects of the effects of exercise and diet on human subjects, usually student volunteers, and require the collection of blood and urine samples to analyze for metabolites and proteins. With the increa...

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