Enzymes, i n vol ved i n t he met abol i sm of gl ucose and f at t y aci ds and i n energy ho meost asi s, ar e cr ucial i n mai nt ai ni ng l i f e, and ma y be var i abl e amon g ani mal speci es, t i ssues, and var i ous ph ys i ol ogi cal and nut r i t ional st at us of a gi ven ani mal . In par t i cul ar, el evat i on s i n MDH act i vi t y and MDH/ LDH ( M/ L) r at i o l i kel y i ndi cat e hei ght ened energy met abol i sm, as MDH of t he mal at e -aspar t at e shut t l e, cont r i butes t o the coupl i ng of gl yc ol ysi s wi t h AT P pr oduct i on vi a t he t r anspor t of cyt osol i c NADH i nt o mi t ochondr i a. T he mal at e -aspar t at e shut t l e, one of t he NAD H s hut t l es, t r ansport s cyt osol i c NADH, a ke y mol ecul e f or oxi dat i ve met abol i s m an d AT P pr oduct i on, i nt o mi t ochondr i a, and pl ays an i mpor t ant r ol e i n t he r egul at i on of mi t ochondr i al met abol i sm i n ani mal cel l s .
Enzyme act i vi t i es of energy met abol i sm, such as MDH, M/ L r at i o, gl ut a mat e dehydr o gen ase ( GLDH) , and f r uct oki nase ( FK ) of t he fel i ne l eukocyt es wer e compar ed a gai nst t hose of t he cani ne leukoc yt es. Si gni f i cant l y l ower MDH, M/ L r ati o, GLDH, an d h i gher FK , pyr u vat e ki nase , and G6P D ma y r ef l ect t he uni que de mands and usages of n ut r i ent s and energy sour ces i n cat s wi t h hi gher i nci dence of obesit y, i nsul i n r esi st ance, and di abet es mel l i t us co mpar ed t o dogs.
El evat i ons i n cyt osol i c and mi t ochondr i al MD H and t he r esul t ant cyt osol i c M/ L r at i o are not ed i n cel l s t hat ma y be expe r i enci ng i ncr eased mi t ochondr i al ATP pr oduct i ons . El evat ed AT P pr oduct i ons, in t ur n, pr ovi de f or i ncr eased energy demands ( i nt ense exer ci se, neopl asti c cell gr owt h, acut e wei ght gai n) . Type 1 di abet i c dogs wi t h hi gher pl asma gl ucose concent r at i ons, showed l ower l evel s MDH, M/ L r at i o, as wel l as AST, as compar ed t o t he heal t hy cont r ol dogs. T hese r ef l ect ed t he def ect i n gl ucose usage and upt ake as energy sour ce i n t he per i pher al ti ssu es, r esul t i ng i n i ncr eased ci r culat i ng pl asma gl ucose c o n c e n t r a t i o n s s e e n i n d i a b e t i c p a t i e n t s . D e c r e a s e d a c t i v i t y l e v e l s o f M A s h u t t l e enzymes ma y be o ne of t he char act eri st ics of energy met abol i sm i n di abet i c dogs, and ma y be usef ul as a di agnost i c i ndi cat or t o moni t or t he over al l met abol i c condi t i on of t he di abet i c pat ient s , i denti f y t hose at r i sk, and pr edi ct t he tr eat ment out come. Racehor ses demo nst r at ed hi gher pl asma MDH and LD H act i vi t i es, and
si gni f i cant l y hi gher p l asma M/ L r at i o as compar ed t o t hose of t he r i di ng hor ses.
Mor eover, r acehor ses demonst r at ed si gni f i cantl y hi gher l evel s of l i pi d met abol i t es as compa r ed t o t hose seen i n ridi ng hor ses. Racehor ses , under i nt ense t r ai ni ng, ma y ha ve ad apt ed t o t he demands of hi gher act i vi t y l evel s by i nc r easi ng muscl e mi t ochondr i al r espi r at i on, oxi dat i ve capaci t y, and f at ut i l i zat i on of t he skel et al muscl es as energy sour ce i n or der t o pr ocess and consume energy mor e eff i ci entl y. In exper i ment al l y o ver f ed do gs wi t h acut e wei ght gai n, si gni f i cant e l e v a t i o n s w e r e n o t e d i n l i p i d m e t a b o l i t e s , g l u c o s e , l e u k o c y t i c M D H a n d L D H . A l t h o u g h n o t s i g n i f i c a n t , p l a s m a a n d l e u k o c y t i c M / L r a t i o s s h o w e d m i l d i n c r e a s e i n t h e o v e r - f e d g r o u p a f t e r t h e f e e d i n g t r i a l . In c o n c l u s i o n , M / L r a t i o i n p l a s m a a n d l e u k o c y t e s r e f l e c t e d p o s i t i v e e n e r g y b a l a n c e a n d i n c r e a s e i n e n e r g y m e t a b o l i s m a s s o c i a t e d w i t h o v e r f e e d i n g .
A bet t er gr asp of t r ends i n shi f t s of t he energy met abol i sm en zymes ma y hel p under st and t he speci es -speci es di ff er ences i n energy pr oduc t i on and usage, and ear l y det ect i on and pr event i on of energy met abol i s m d ysr e gul at i on s such as di abet es, neopl asi a, and obesi t y. Fur t her st udi es on var i ous di sease model s and met abol i c st at es i n diff er ent speci es ma y a ssi st i n a bet t er underst andi ng of t hei r cl i ni cal usage.
