Nonprotein

Nonprotein refers to any substance that is not a protein, or more specifically, to nitrogen-containing compounds that are not polypeptide chains. In biochemistry and nutrition, nonprotein nitrogen (NPN) denotes nitrogenous compounds that are not proteins but still contribute to total nitrogen in a sample.

Common examples of NPN include urea, uric acid, creatinine, ammonia, and various soluble amines, as well as nucleotides and other small nitrogenous molecules. These compounds can be present in blood, urine, tissues, and in the environment of plants and animals. The term is often used in contrast to true protein, which consists of long chains of amino acids folded into functional structures.

Biological and agricultural relevance varies by context. In ruminant nutrition, NPN such as urea can serve as a nitrogen source for rumen microbes, enabling microbial protein synthesis and supporting animal growth when dietary protein is limited. In soil science, NPN components contribute to total soil nitrogen and can influence microbial activity and nutrient cycling, eventually converting to inorganic forms usable by plants. The NPN concept is also important in protein quantification; crude protein estimates from nitrogen content assume all nitrogen derives from proteins, so high NPN can lead to overestimation of actual protein.

Analytically, distinguishing protein nitrogen from NPN can be important for assessing nitrogen balance, kidney function, or dietary quality. Techniques range from traditional Kjeldahl methods to modern assays that target or exclude specific nitrogenous species.