Thank you for referring me to the letter from Dr Mats Enlund concerning the enzyme adenylate kinase (AK), a well validated marker of brain damage measured in cerebrospinal fluid(CSF).
We thank Dr Mats Enlund for his comments after reading our paper . We agree that AK has been used as a marker of brain damage in a great number of different pathological situations. Brain tissue AK activities have been used to assess the degree of brain injury and the protective effects of hypothermia and antioxidant [2,3]. However, as Dr Enlund mentions in his letter, almost all publications available are from CSF-AK analysis. To our knowledge, there are no publications on plasma AK analysis. Ideally, for clinical purposes, it is necessary to have a biochemical marker that can be measured in blood.
Cerebrospinal fluid (CSF) concentration of adenylate kinase
Following cardiopulmonary bypass (CPB) surgery
A significant increase in CSF-AK was correlated with changes in an index of intellectual function  and psychometric performance . Its measurement might therefore be useful in research to improve the quality of open-heart surgery. Magnetic resonance imaging (MRI) is a sensitive measure of subclinical cerebral ischaemia after CPB. CSF neurone-specific enolase (NSE) and lactic dehydrogenase may be less sensitive than MRI, but appear to be more sensitive than CSF-AK . When CSF-AK was used to assess cerebral ischaemia of gaseous microembolic origin, there was no significant difference in AK as marker of ischaemia between nitrous oxide and control groups following CPB .
Following stroke and global cerebral ischaemia
CSF-AK levels were correlated with clinically neurological signs and sizes of infarction from computed tomography (CT) [8-10] following acute cerebral ischaemic infarction. However, the results were unrepeatable because of difficulties in sampling and handling CSF material . The contamination of AK from erythrocytes and serum  is a significant limiting factor for AK to fulfil the criteria for an ideal CSF marker. Nevertheless, this limiting factor could be ruled out by using the specific light absorbency for oxyhaemoglobin .
Following cardiac arrest
The CSF-AK activity at 24 h after cardiac arrest showed predictive prognosis in 12 patients, however, CSF-AK did not show any prognostic value in neurological outcome after cardiac arrest in 32 patients in Edgren's study.
In summary, CSF-AK is an interesting and important biochemical marker of cerebral damage. However, serum AK levels may be difficult to interpret because of contamination by erythrocyte-derived AK.
D. N. F. HARRIS
Department of Anaesthesia and Intensive Care
Birmingham Heartlands Hospital; Birmingham, UK
Hammersmith Hospital; London, UK
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2Tokuda Y, Uozumi T, Kawasaki T. The superoxide dismutase activities of cerebral tissues, assessed by the chemiluminescence methods, in the gerbil focal ischemia/reperfusion and global ischemia models. Neurochem Int
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