Bauer C., Cercignani G., Paolini M. Defining "enzyme unit" correctly. In: Lancet, vol. 348 (9022) pp. 271 - 271. Elsevier, 1996. |

Abstract (English) |
If we take the definition of the international enzyme unit (IU) as that amount of enzyme that catalyses transformation of 1 μmol of substrate per minute (under conditions of substrate saturation and optimum pH, &c) one would expect that 1 IU of, for example, lactate dehydrogenase (LDH) should catalyse the dehydrogenation of 1 μmol per minute of lactate in the above conditions. This is, however, incorrect: 1 IU of LDH is usually meant as the amount that catalyses the conversion of 1 μmol of pyruvate per minute, which is a much lower amount of enzyme than that required to oxidise 1 μmol of lactate per minute to pyruvate; this is attributable to the fact that reaction rates are different (at the same concentration of reagents) in opposite directions, taking into account the negative change in free energy (δG°′) in the direction of lactate (δG°′= −6 kcal/mol). Hence the enzyme unit is different in opposite directions-there is a right or forward unit, and a left or backward unit. Ambiguity is derived from the fact that an enzyme is usually named in one direction (and often the one with δG°′ > > 0, as for LDH and some kinases), whereas the unit is usually defined in the opposite direction—namely, that which favours the reaction. We suggest that the substrate used in the definition of enzyme unit (usually that with δG°′ > > 0) should be clearly mentioned, keeping well in mind that the opposite unit (in the direction with δG°′ > > 0) is a greater amount of enzyme. The two opposite units may coincide only in the special case where δG°′ is roughly equal to 0 (eg, 1 IU aspartate aminotransferase ·1 IU glutamate-oxaloacetate transaminase). In clinical biochemistry measures such as these could be very important. Also, for enzymes catalysing reactions with equilibrium constant (K′eq) close to 1, appreciable differences could be achieved by doing the assay in two directions. Considering, for example, alanine aminotransferase (K′eq=2·22), if we measure 20 U/L in one direction, in the opposite direction we will obtain 44 U/L and a typical borderline case could be suspected; again, considering aspartate aminotransferase, where K′eq =0·16-0·17, such an effect could be more relevant. We suggest that the substrate used in the definition of enzyme unit (usually that with ΔG°′much less-than0) should be clearly mentioned, keeping well in mind that the opposite unit (in the direction with ΔG°′much greater-than0) is a greater amount of enzyme. The two opposite units may coincide only in the special case where ΔG°′ is roughly equal to 0 (eg, 1 IU aspartate aminotransferase congruent with1 IU glutamate-oxaloacetate transaminase). In clinical biochemistry measures such as these could be very important. Also, for enzymes catalysing reactions with equilibrium constant (K′eq) close to 1, appreciable differences could be achieved by doing the assay in two directions. Considering, for example, alanine aminotransferase (K′eq =2·2), if we measure 20 U/L in one direction, in the opposite direction we will obtain 44 U/L and typical borderline case could be suspected; again, considering aspartate aminotransferase, where K′eq=0·16-0·17, such an effect could be more relevant. | |

DOI: | 10.1016/S0140-6736(05)65584-5 | |

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