An attempt to determine the tissue origin of equine serum alkaline phosphatase by isoelectric focusing.

The main purpose of this study was to ascertain whether isoelectric point determination of alkaline phosphatase (AP) using an isoelectric focusing technique on agarose gels could define the isoenzymes present in healthy equine serum. The isoelectric points of AP extracted from nine tissues ranged from pH 3.5 to 7.5 with all tissues having multiple bands. There was considerable similarity in band pattern among tissues, with only pancreatic and colostral AP having substantially different isoelectric points from the others. Sera contained thirteen bands with isoelectric points ranging from pH 3.5 to 6.2 and as each band was common to more than one tissue it was not possible to define the tissue origin of these by direct comparison with tissue patterns. The intensity of all serum bands declined as foals aged, with the greatest decrease in bands 4 and 5 (numbered from the anode). There was no relative change in the banding pattern between early and late pregnant mares or in the sera of two foals before and after ingestion of colostrum. The mean (+/- SD) total serum AP activities of young foals (1676 +/- 1100 IU/L), three month foals (402 +/- 64 IU/L) early pregnant (190 +/- 54 IU/L) and late pregnant mares (109 +/- 26 IU/L) were significantly different from each other whereas colostral ingestion in two neonatal foals had no effect. We concluded that equine AP is a very heterogeneous protein and that normal horse sera do not contain significant renal or small intestinal derived AP. However isoelectric focusing alone could not differentiate bone from liver derived AP in sera.