suffered severe deterioration of vision before being discovered. Even when patients are originally thought to have escaped ophthalmic complications, unsuspected glaucoma may yet be present with slowly advancing loss of sight. Every patient with facial naevus should have regular ophthalmic supervision.

The isoenzymes ADA1 and ADA2 of the enzyme adenosine deaminase (ADA 3.5.4.4) deaminate mainly two nucleosides: adenosine and 2'deoxyadenosine, producing inosine and 2'deoxyinosine. Adenosine and 2'deoxyadenosine are molecules with many effects on human cells [1][2][3][4]. Thus, the homeostasis of these substances and the activity of the isoenzymes ADA1 and ADA2 in human cells are of extreme importance. Isoenzyme ADA1In humans, the isoenzyme ADA1 is ubiquitous and guarantees the downregulation of the substrates adenosine and 2'deoxyadenosine [1]. This homeostasis is particularly important because a low level of 2'deoxyadenosine is essential for a proper function in immune cells [2][3][4][5]. The isoenzyme ADA1 is also present in red cells, which are equipped with an efficient mechanism to capture and internalize 2'deoxyadenosine. Therefore, in humans, red cells form a "dialysis" system for circulating 2'deoxyadenosine and, in this way, preserve the nucleated cells from an excessive external supply of 2'deoxyadenosine [6].The importance of ADA1 in cells is revealed by the disfunction of the immune response in subjects congenitally lacking ADA1 due to inadequate homeostasis of 2'deoxyadenosine in their immune cells [2,4,6]. There are several other important effects of the substrate adenosine which also bear witness to the importance of the presence of ADA1 in the human body [7]; these will not be discussed here.The Km of ADA1 is 5.2×10 -5 M. ADA1 has an optimal pH of 7-7.5 and a similar affinity for both adenosine and 2'deoxyadenosine (2'deoxyadenosine/adenosine deaminase ratio of 0.75) [1,8].These features make ADA1 highly efficient in deaminating the substrates (adenosine and 2'deoxyadenosine) in biological sites where the pH is optimal for this isoenzyme even though the substrate concentration is very low. Isoenzyme ADA2 (ADA1-ADA2 isoenzymatic system)The isoenzyme ADA2 is not ubiquitous, but coexists with ADA1 only in monocytes-macrophages. ADA2 and ADA1 are coded by different gene loci [1,4,9,10].The Km of ADA2 is 200×l0 -5 M; ADA2 has an optimum pH of 6.5 and a weak affinity for 2'deoxyadenosine (2'deoxyadenosine/adenosine deaminase ratio of 0.25) [1,8,9]. These features make ADA2 inefficient in deaminating 2'deoxyadenosine in biological sites where the pH is higher than the optimum for this isoenzyme and the concentration of substrates is too low.Why, therefore, should ADA2 be present in monocytesmacrophages, where conditions are not suitable for its optimum function? To answer this question, which is extremely important from a biological point of view, it is necessary to consider ADA1 and ADA2 as a system which acts to guarantee the homeostasis of adenosine and 2'deoxyadenosine in monocytes-macrophages. This homeostatic mechanism involves two substrates and two isoenzymes. Both isoenzymes have similar affinity for the substrate adenosine, whilst ADA2 has a different affinity (very weak) for the substrate 2'deoxyadenosine [1,8].Through the simulation model STELLA II (High Performance System Inc., 1993), we h...

Some properties of human serum and human tissue adenosine deaminase have been studied. Normal and pathological human serum adenosine deaminase and normal human tissue adenosine deaminase show the same activation energy values which are different from those found for the calf intestine enzyme. In the human species, but not in the bovine species, the serum enzyme differs from the tissue enzyme in pH optimum and relative substrate specificity for adenosine and 2’deoxyadenosine. Temperature conversion factors for human serum and calf intestine adenosine deaminase have also been calculated.

Cerebrospinal fluid adenosine deaminase activity was assayed in 13 cases of tuberculous meningitis. In all the cases, enzymatic activity was high but subsequently showed a progressive decrease, paralleled by clinical recovery and cere- brospinal fluid cellular and biochemical data normalization. Values scarcely higher than normal were reported in cases of bacterial and viral meningitis. In cases of various acute and chronic inflammatory, degenerative, vascular and neoplastic diseases of the nervous system, cerebrospinal fluid adenosine deaminase activity did not differ significantly from that of normal subjects. The assay of cerebrospinal fluid adenosine deaminase activity seems to be of considerable practical interest with regard to the differential aetiologic diagnosis between tuberculous and viral lymphocytic meningitis.

No abstract