The apical surface of proximal tubular epithelial cells contains numerous microvilli that form the brush border and contain proteins with enzymatic functions to carry out the specialized tasks of the proximal tubule. Intracellular enzymes can be released into the urine with injury [2
] either by exocytosis or leakage. The proteins can exist in the free form or may be membrane-encased as exosomes. Several different classes of enzymes can be found: lysosomal proteins, such as N-acetyl-β-D
-glucosaminidase (NAG), brush border enzymes including gamma-glutamyl transferase (GGT) and alkaline phosphatase, as well as cytosolic proteins such as α-glutathione S-transferase (α-GST). Furthermore, when proximal tubular epithelial cells are injured, they may not metabolize cystatin C properly, and filtered intact cystatin C may appear in the urine. Similarly, injured cells may not completely reabsorb low-molecular-weight proteins that are freely filtered into the urinary space, such as α1
- and β2
Westhuyzen et al.[3
] compared the predictive value of a number of tubular enzymes for the subsequent development of AKI, defined as a 50% rise in serum creatinine to at least 1.7 mg/dl. Four of 26 subjects developed AKI; baseline levels of GGT, AP, NAG, α-GST and π-GST were higher in those who developed AKI, compared to those who did not. α-GGT and π-GST had the best predictive value on their own, with areas under the receiver-operating characteristic curve (AUC-ROC) of 0.95 (95% CI 0.79–1.0) and 0.93 (95% CI 0.74–1.0), respectively. Changes in enzyme levels preceded detectable changes in timed creatinine clearance. However, when the authors attempted to develop cutpoints based on this small study and tested the generalizability of their results in a test population of 19 patients (4 of whom developed AKI), the sensitivity and specificity of these biomarkers were significantly reduced.
Several investigators have examined the ability of tubular enzymes to predict adverse clinical outcomes. Herget-Rosenthal et al.[4
] risk-stratified patients with nonoliguric AKI (defined as a doubling in creatinine from a baseline concentration of <106 μmol/l to at least 115 μmol/l) using tubular enzymes as biomarkers. They identified 73 subjects who met prespecified criteria for AKI; 26 of these individuals subsequently required dialysis. They measured urinary excretion of cystatin C, α1
- and β2
-microglobulin, α-GST, NAG, retinol-binding protein (RBP), GGT and lactate dehydrogenase on the day of study enrollment. Cystatin C and α1
-microglobulin (markers of abnormal proximal tubule function) had the best predictive value for the need for dialysis, with AUC-ROC curves of 0.92 and 0.86, respectively. Of the tubular enzymes studied, NAG had the best predictive value, with an AUC-ROC of 0.81. In another study, Chew et al. [5
] found that levels of NAG and tissue non-specific alkaline phosphatase were higher in AKI patients with poor outcomes (defined as need for dialysis or death). Liangos et al. [6
] recently performed a study of NAG and kidney injury molecule-1 (KIM-1, a tubular injury marker, discussed below) in 201 patients with established AKI. They found that elevated NAG levels portended poor clinical outcomes, with the odds of death or dialysis requirement increased over five-fold in patients with the highest versus lowest quartiles or urinary NAG levels, even after careful multivariable adjustment for disease severity and comorbidity. The predictive power of KIM-1 levels in this study will be discussed below.
Tubular enzymes present in the urine have long been studied as markers of AKI, but they have not been adopted in widespread clinical use either as early diagnostic tests, prognostic indicators, or surrogate endpoints for interventional studies. Some authors have suggested that tubular enzymes are overly sensitive, because they tend to rise after injuries such as cardiopulmonary bypass without an associated rise in SCr [7
]. Investigators should exercise caution, however, in interpreting performance characteristics of new biomarkers against a gold standard like SCr that has poor specificity and sensitivity: cardiac troponin would appear to be nonspecific against earlier (and now discredited) cardiac biomarkers like lactate dehydrogenase.